Everything used to be fine (iTunes 6), but with the latest upgrade, iTunes pegs the processor(s) at 100%, becomes very sluggish, won't respond to mouse or keyboard, etc. If a song is playing, it manages to keep playing the song without skipping, but that's about it.
At first I thought my WinXP system might be to blame: At almost 4 years old, I assumed it was time for the customary blow-away-your-entire-configuration-and-rebuild-from-scratch that Windows systems generally need every few years.
Luckily, having just built up a fresh Vista system, I simply installed iTunes over there. iTunes still performed horribly. A quick Google search indicates that I am not alone in my suffering.
I noticed that performance was really bad when it first imported my media library due to iTunes' gapless playback analysis. But that's all finished now, and iTunes is still messed up.
Based on Zack's recommendation, I'm going to give Media Monkey a try. I've become pretty dependent on iTunes over the last few years, but no matter how great it is, if I can't use it then it's worthless to me!