Hello Thierry,
 
  I will try to help showing some information gathered from my lab server as you requested:
 
- Enviroment:
 
 10 x Ubuntu 10.04 x86_64 with several services (SMTP, MySQL, Oracle, DB2, TeamSpeak servers, Tomcats, bind ... etc).
 1 x Windows 7 x86_64
 3 x Debian Lenny x86_64 (iscsitarget, pacemaker cluster testing)
 
 RAM schema (assigned to the VMs): 3 vms 128 MB, 6 vms 256 MB, 2 vms 512 MB, 1 vm 768 MB, 2 vms 1GB
 SMP: 5 vms with 2 SMP (num vCPU) configured, rest 1 SMP
 
- Hardware:
 
 1 x Intel(R) Core(TM)2 Quad CPU           @ 2.66GHz
 8 GB RAM DDR2 800 Mhz / 25 ns

 ---------------------------------------------

* Summary:

Usefull information:
  CPU % usage goes stable, also memory is examined and released after the 3rd full_scans
  CPU % usage is specially high between 1st and 2nd scan (when ksm is merging pages at the same time that it is doing the second scan)
  Memory starts being released after the 1st scan is completly and the 2nd is in progress (so past some minutes)
 
KSM default pages_to_scan:100 sleep_millisecs:20 CPU: ~11,67% Time_for_a_full_scan: 3m35.209s
KSM pages_to_scan:60 sleep_millisecs:40 CPU: ~3,44% Time_for_a_full_scan: 11m50.734s
KSM pages_to_scan:100 sleep_millisecs:50 CPU: ~4,21% Time_for_a_full_scan: 8m55.533s
KSM pages_to_scan:60 sleep_millisecs:60 CPU: ~2,00% Time_for_a_full_scan: 17m46.094s
KSM pages_to_scan:100 sleep_millisecs:120 CPU: ~1,67% Time_for_a_full_scan: 21m26.285s
 
Mem Optimization in all the tests (there before or after):
  RAM available (reminder): 8 GB
  Memory optimizacition thanks to ksm: 14-16% full available memory (~ 1.3 / 1.1 Gb of more free memory)

Conclusions:
  ksm is good but need some tweaks to doesnt become a problem more than a help
  I'd consider to off ksm in /etc/default/qemu-kvm for desktop versions of Ubuntu (availabe but off by default)
  I'd consider to switch default value of sleep_millisecs: 120 even a bit higher in the ubuntu server version. More conservative...
 
 -------------------------------------------
 
 
 root@core:/sys/kernel/mm/ksm# echo 2 > /sys/kernel/mm/ksm/run
 root@core:/sys/kernel/mm/ksm# sleep 30
 root@core:/sys/kernel/mm/ksm# landscape-sysinfo 
   System load:     1.02              IP address for lo:     xxx
   Usage of /home:  66.1% of 2.68TB   IP address for eth1:   xxx
   Memory usage:    60%               IP address for eth1:0: xxx
   Swap usage:      0%                IP address for bond0:  xxx
   Processes:       199               IP address for br0:    xxx
   Users logged in: 1
 root@core:/sys/kernel/mm/ksm# echo 1 > /sys/kernel/mm/ksm/run ; time /tmp/tmp.bash
 real    3m35.209s
 user    0m0.070s
 sys     0m0.130s
 [... /tmp/tmp.bash
     #!/bin/bash 

     while true
     do
       SALIDA=`cat /sys/kernel/mm/ksm/full_scans`
       if [ $SALIDA == "1" ]
       then
          break
       else
          sleep 5
       fi
     done
 .... yeee... terrible but enough for a script done in 10 secs...]
 [...waiting for a pair of aditional scans...]
 root@core:/sys/kernel/mm/ksm# cat full_scans
 3
 root@core:/sys/kernel/mm/ksm# ps axuw | grep ksm | grep -v grep 
 root        53 10.7  0.0      0     0 ?        SN   17:51  12:08  \_ [ksmd]
 
 (Cycle/scan time is 3m35secs ~= 215 secs -- doing check each 2 secs so I'll use "iostat -p <pid> 2 215/2")
 
 root@core:/sys/kernel/mm/ksm# pidstat -p 53 2 108
 Linux 2.6.32-24-server (core)   18/08/10        _x86_64_        (4 CPU)
 [...]
 Media:             53    0,00   11,67    0,00   11,67     -  ksmd

 root@core:/sys/kernel/mm/ksm# cat full_scans 
 4
 root@core:/sys/kernel/mm/ksm# cat max_kernel_pages 
 511518
 root@core:/sys/kernel/mm/ksm# cat pages_shared 
 42045
 root@core:/sys/kernel/mm/ksm# cat pages_sharing 
 287118
 root@core:/sys/kernel/mm/ksm# cat pages_to_scan 
 100
 root@core:/sys/kernel/mm/ksm# cat pages_unshared 
 622240
 root@core:/sys/kernel/mm/ksm# cat pages_volatile 
 97214
 root@core:/sys/kernel/mm/ksm# cat run 
 1
 root@core:/sys/kernel/mm/ksm# cat sleep_millisecs 
 20

 root@core:/sys/kernel/mm/ksm# cat /proc/meminfo 
 MemTotal:        8194212 kB
 MemFree:         1000880 kB
 Buffers:          432396 kB
 Cached:          2865740 kB
 SwapCached:            0 kB
 [...]
 root@core:/sys/kernel/mm/ksm# landscape-sysinfo 
  System load:     6.44              IP address for lo:     xxx
  Usage of /home:  66.1% of 2.68TB   IP address for eth1:   xxx
  Memory usage:    44%               IP address for eth1:0: xxx
  Swap usage:      0%                IP address for bond0:  xxx
  Processes:       199               IP address for br0:    xxx
  Users logged in: 1

------------------------------------------------------------
 
 root@core:/sys/kernel/mm/ksm# echo 2 > /sys/kernel/mm/ksm/run
 root@core:/sys/kernel/mm/ksm# sleep 30
 root@core:/sys/kernel/mm/ksm# echo 40 > /sys/kernel/mm/ksm/sleep_millisecs
 root@core:/sys/kernel/mm/ksm# echo 60 > /sys/kernel/mm/ksm/pages_to_scan
 root@core:/sys/kernel/mm/ksm# echo 1 > /sys/kernel/mm/ksm/run ; time /tmp/tmp.bash
 real    11m50.734s
 user    0m0.270s
 sys     0m0.390s
 [...waiting for a pair of aditional scan...]
 root@core:/sys/kernel/mm/ksm# cat full_scans 
 3
 root@core:/sys/kernel/mm/ksm# pidstat -p 53 2 355
 Linux 2.6.32-24-server (core)   19/08/10        _x86_64_        (4 CPU)
 
 00:05:57          PID    %usr %system  %guest    %CPU   CPU  Command
 00:05:59           53    0,00    0,00    0,00    0,00     0  ksmd
 00:06:01           53    0,00    4,00    0,00    4,00     0  ksmd
 [...]
 00:17:45           53    0,00    1,50    0,00    1,50     0  ksmd
 00:17:47           53    0,00    0,50    0,00    0,50     0  ksmd
 Media:             53    0,00    3,44    0,00    3,44     -  ksmd
 root@core:/sys/kernel/mm/ksm# cat full_scans 
 4
 root@core:/sys/kernel/mm/ksm# cat /proc/meminfo 
 MemTotal:        8194212 kB
 MemFree:         1027900 kB
 Buffers:          478248 kB
 Cached:          2684208 kB
 SwapCached:         1020 kB

------------------------------------------------------------

 root@core:/sys/kernel/mm/ksm# echo 2 > /sys/kernel/mm/ksm/run
 root@core:/sys/kernel/mm/ksm# echo 50 > /sys/kernel/mm/ksm/sleep_millisecs
 root@core:/sys/kernel/mm/ksm# echo 100 > /sys/kernel/mm/ksm/pages_to_scan
 root@core:/sys/kernel/mm/ksm# cat /proc/meminfo 
 MemTotal:        8194212 kB
 MemFree:           59480 kB
 Buffers:          485952 kB
 Cached:          2636984 kB
 SwapCached:         1120 kB
 [...]
 root@core:/sys/kernel/mm/ksm# echo 1 > run; time /tmp/tmp.sh                             
 real    8m55.533s
 user    0m0.180s
 sys     0m0.380s
 [...waiting for a pair of aditional scans...]
 root@core:/sys/kernel/mm/ksm# cat full_scans 
 3
 root@core:/sys/kernel/mm/ksm# cat /proc/meminfo   
 MemTotal:        8194212 kB
 MemFree:         1026668 kB
 Buffers:          482376 kB
 Cached:          2642836 kB
 SwapCached:         1316 kB
 root@core:/sys/kernel/mm/ksm# pidstat -p 53 2 267
 Linux 2.6.32-24-server (core)   19/08/10        _x86_64_        (4 CPU)
 
 00:40:40          PID    %usr %system  %guest    %CPU   CPU  Command
 00:40:42           53    0,00    1,50    0,00    1,50     2  ksmd
 00:40:44           53    0,00   11,00    0,00   11,00     2  ksmd
 [...]
 00:49:32           53    0,00    6,00    0,00    6,00     2  ksmd
 00:49:34           53    0,00    6,00    0,00    6,00     2  ksmd
 Media:             53    0,00    4,21    0,00    4,21     -  ksmd
 root@core:/sys/kernel/mm/ksm# cat full_scans 
 4
 
 ------------------------------------------------------------
 root@core:/sys/kernel/mm/ksm# echo 2 > /sys/kernel/mm/ksm/run
 root@core:/sys/kernel/mm/ksm# echo 60 > /sys/kernel/mm/ksm/sleep_millisecs 
 root@core:/sys/kernel/mm/ksm# echo 60 > /sys/kernel/mm/ksm/pages_to_scan 
 root@core:/sys/kernel/mm/ksm# cat /proc/meminfo 
 MemTotal:        8194212 kB
 MemFree:           63684 kB
 Buffers:          501664 kB
 Cached:          2606948 kB
 SwapCached:         1316 kB
 [...]
 root@core:/sys/kernel/mm/ksm# echo 1 > run; time /tmp/tmp.sh
 real    17m46.094s
 user    0m0.400s
 sys     0m0.540s
 [...waiting for a pair of aditional scans...]
 root@core:/sys/kernel/mm/ksm# cat full_scans 
 3
 root@core:/sys/kernel/mm/ksm# pidstat -p 53 2 533
 Linux 2.6.32-24-server (core)   19/08/10        _x86_64_        (4 CPU)
 
 01:48:30          PID    %usr %system  %guest    %CPU   CPU  Command
 01:48:32           53    0,00    0,00    0,00    0,00     1  ksmd
 01:48:34           53    0,00    3,00    0,00    3,00     2  ksmd
 [...]
 02:06:14           53    0,00    0,50    0,00    0,50     2  ksmd
 02:06:16           53    0,00    3,50    0,00    3,50     2  ksmd
 Media:             53    0,00    2,00    0,00    2,00     -  ksmd
 root@core:/sys/kernel/mm/ksm# cat full_scans 
 4
 root@core:/sys/kernel/mm/ksm# cat /proc/meminfo 
 MemTotal:        8194212 kB
 MemFree:          938949 kB
 Buffers:          525132 kB
 Cached:          2615487 kB
 SwapCached:         1184 kB

 ------------------------------------------------------------
 root@core:/sys/kernel/mm/ksm# echo 2 > /sys/kernel/mm/ksm/run
 root@core:/sys/kernel/mm/ksm# echo 120 > /sys/kernel/mm/ksm/sleep_millisecs  
 root@core:/sys/kernel/mm/ksm# echo 100 > /sys/kernel/mm/ksm/pages_to_scan  
 root@core:/sys/kernel/mm/ksm# cat /proc/meminfo                             
 MemTotal:        8194212 kB
 MemFree:           58696 kB
 Buffers:          464916 kB
 Cached:          2637064 kB
 SwapCached:         1184 kB
 [...]
 root@core:/sys/kernel/mm/ksm# landscape-sysinfo                             
  System load:     2.44              IP address for lo:     xxx
  Usage of /home:  66.1% of 2.68TB   IP address for eth1:   xxx
  Memory usage:    64%               IP address for eth1:0: xxx
  Swap usage:      0%                IP address for bond0:  xxx
  Processes:       204               IP address for br0:    xxx
  Users logged in: 1

 root@core:/sys/kernel/mm/ksm# echo 1 > run; time /tmp/tmp.sh
 real    21m26.285s
 user    0m0.520s
 sys     0m0.990s
 [...waiting for a pair of aditional scans...]
 root@core:/sys/kernel/mm/ksm# cat full_scans 
 2
 root@core:/sys/kernel/mm/ksm# pidstat -p 53 2 643
 Linux 2.6.32-24-server (core)   19/08/10        _x86_64_        (4 CPU)
 
 03:17:21          PID    %usr %system  %guest    %CPU   CPU  Command
 03:17:23           53    0,00    2,00    0,00    2,00     0  ksmd
 03:17:25           53    0,00    0,00    0,00    0,00     0  ksmd
 [...]
 03:38:45           53    0,00    1,50    0,00    1,50     0  ksmd
 03:38:47           53    0,00    0,00    0,00    0,00     0  ksmd
 Media:             53    0,00    1,67    0,00    1,67     -  ksmd
 root@core:/sys/kernel/mm/ksm# cat full_scans 
 3
 root@core:/sys/kernel/mm/ksm# pidstat -p 53 2 643
 Linux 2.6.32-24-server (core)   19/08/10        _x86_64_        (4 CPU)
 
 03:38:47          PID    %usr %system  %guest    %CPU   CPU  Command
 03:38:49           53    0,00    0,00    0,00    0,00     0  ksmd
 03:38:51           53    0,00    0,50    0,00    0,50     0  ksmd
 [...]
 04:00:13           53    0,00    0,00    0,00    0,00     0  ksmd
 Media:             53    0,00    1,67    0,00    1,67     -  ksmd
 root@core:/sys/kernel/mm/ksm# cat full_scans
 4