I hear you Andrew. I want so much for FCPX to work well. I like the interface, the video scrubbing feature, and actually enjoy using it. I just get stuttering playback, I guess because of the background rendering (?). I'm using a 2011 15" Macbook Pro with top specs (best video card apple offers for that computer, 8gb of RAM, etc.), but still get the playback issue and spinning beachball lag. It does crash on occassion for me as well. I'm curious about something... Have you noticed that in demo videos (e.g., for training, effect plug-ins, etc.) FCPX seems to be responding nicely, and not acting like a beta program? I wonder if these people are using a Mac Pro, or an iMac for that matter. Seems like most people I've read online who are using the MacBook Pro are consistently experiencing these symptoms. I could be wrong. I'm holding on; I've purchased Premiere 5.5 and actually prefer FCPX, when it's working properly, but Apple's taking it's time to optimize this program!
Seems like the selling point of FCPX using all of the computer's resources (e.g., RAM, processing power, etc.) may be hindering things somehow. I notice, like others on this forum, that the RAM is hogged up, as a precurser to things acting funny in the program.
Anyway, all that to say, I feel your pain. I'm using this program now, even with its flaws, so that when Apple fixes its shortcomings, I'll have a good handle on how to use it (keyboard shortcuts and all).