I have problems with iTunes for Windows because the QuickTime platform isn't the native way to handle media on a Windows box (which it is in OS X). So already you're dealing with a beast that's more complicated than it should be.

That said, I use iTunes on my Windows PC. It's not great for large libraries or doing heavy I/O stuff (changing meta-data for multiple long movies, for instance), but it gets the job done and it's a pretty seamless experience between the music player, the store and the iPod/iPhone.

It's good, but in my opinion it needs a lot of work making it more responsive and efficient