I think, the following feature would be nice:
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In a few cases Mixxx doesn't recognize any or the right BPM-Rate. Then you have two possibilities to correct, as I see. Figure out the rate and write them manually into the Metadata or use the Tap-Button. I don't know, how other guys think, but I can't handle tap very well and manuelly figure out takes lot of time.
Very often you see the beat in the waveform or know where the BPM line have to be. Maybe it is possible to create an visual editor (for example in the analyse view) where you see the waveform and can correct the BPM-lines with the mouse (maybe with control-"left mouse button"). I think, that gives an very intuitive and fast method to correct or create an BPM-Rate, that the automatic algorithm cannot deal with.
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See also here: http://mixxx.org/forums/viewtopic.php?f=1&t=3363
Mixxx 1.11 will have a much better BPM and beat detection. The beat grids will be correct in most cases unless the track does not follow a regular 4/4 pattern.
Personally, I think there's little chance to realize a visual editor for BPM correction because you can correct the first beat in the grid quite easily in Mixxx 1.11. And if the beat is not right you can still try to round BPM values like 127.98 or 128.05 to 128. Most modern dance music have an integer BPM. This will result in a perfect grid many times.
But as I said stay tunes for our next release!