My experience is that the bugs in FCPX are vastly better than a year ago.
Like John Davidson I still suffer from quite a few though.
[John Davidson] "I get strange things that happen like if I add a favorite clip and then go into the project timeline and hit spacebar - the favorite still plays in the source thumbnail. I have to select a different clip and shift-f it from the project to get the project to actually play back. "
Yes, this also happens to me but overall with the help of Preference Manager and occasionally Force Quit I tend to have a smooth experience now.
From the posts on the cow, bugs seem very machine and operating system defendant, some people are still dealing with weird occurrences and crashes on a regular basis but I think that more experienced users of FCPX adopt preventative behaviour to avoid problems.
I now have 22Gb Ram in my 2008 Quad Core Mac Pro and that seems to have improved matters. It also seems important to run the latest FCPX version under Lion.
Larry, my name is, Jordan has written an excellent blog resume on maintaining performance:
http://www.larryjordan.biz/fcpx-performance-tips/
