I have Caps Lock bound to Compose (under X; it does shift_lock under the console which I never use). I can successfully enter "é" using <Compose><'><e>, and I can successfully enter "‘" using <Compose> <Shift>-<,> <'>, but I cannot (into any application I've tried, which include Firefox, gnome-terminal, and the text editor) enter the opening double quote ("“"); it's supposed to be doable using <Compose> <Shift>-<,> <Shift>-<'> (see, e.g., the X Compose key map at http://webcvs.freedesktop.org/xorg/xc/nls/Compose/en_US.UTF-8?view=co )
Notwithstanding a few applications are smart enough to figure out which quote I want, it would be very nice to be able to type them. A few other minor lacunae: the degree symbol (<Compose> <o> <o>), the per-mille sign (<Compose> <%> <o>), the musical sharp sign (<Compose> <#> <#>), and the one-half symbol (<Compose> <1> <2>). I don't find myself wishing for any of these last few.
I am using a generic US laptop keyboard, set to Generic 104-key PC and U.S. English, with an option set to turn Caps Lock into Compose, and no other options set.
This looks like a duplicate of #74360.