After using GIMP for a little while now, I can say it was really awkward getting used to the UI. But I'm getting more comfortable with it every time I use it. Actually, now that I've adjusted to the UI and memorized the hotkeys, I've stopped using PSP.
I've been reading up on GIMP 2.8 and it does sound pretty necessary. My main annoyance is the windows floating all over: If I don't have an image maximized and I misclick, I end up focusing a completely different window to foreground. It's not especially detrimental, but it could be better.
I've also been reading up on criticisms of GIMP. Most of the criticisms I've seen have to do with GIMP's lack of capability when it comes to non-destructive editing and >8-bit color channels. For the most part, these limitations wouldn't affect my own usage of GIMP.
Overall, I'm pretty satisfied with it.