This list will document the bugs in Final Cut Pro X as they become apparent.
This one is scary. There are several reports of people losing work due to FCPX's autosave function (which is the only way to save) not working.
When you add media from another drive and choose not to copy the files to your Final Cut Events folder, FCPX adds symbolic links to the Final Cut Events folder that point back to these files.
Some people have been reporting that they relaunched FCPX to find all of their media offline. When they looked in the Final Cut Events folder, FCPX had either deleted the symbolic links or failed to save them in the first place.
You can reconnect them by going to File > Import > Files but this causes another bug to surface: the files will always be copied to your Final Cut Events folder even if you deselect the option to do so.
Some people are reporting that Undo is disabled for them. What makes this an especially big problem is that FCPX saves automatically and so any mistakes they make are immortalized forever if they can't undo them.
There are reports of projects disappearing from the project library.
If you experience problems with media, I would recommend updating to OS X 10.6.8 which "Fixes an issue when importing certain media files into Final Cut Pro." I don't yet know if it solves these particular issues though.
Apple recommends only running FCPX on machines with one graphics card.
This can be caused by templates from previous versions of Motion residing in the ~/Library/Application Support/Motion folder. To get around this, move the templates from this folder. More info here.
This can happen if certain folders exist on the drive. The poster in this forum thread says folders called "private" and possibly "backup" can cause this issue:
I'd be interested to hear if anyone finds any other folders that can cause the drive to disappear in FCPX.
More bugs will be listed as they surface.