To be honest ...
The Good:
1) Games generally seem faster
2) Suppose to have some new capabilities, but I mainly like the new About Phone > Battery Use area
3) My commented that the icons look nicer (But actually I prefer the older ones)
4) Improved text selection tools (but that introduced other problems?)
The Bad:
1) I haven't rooted yet and I'm waiting to find one that works so that I can do a backup of my apps and settings before I factory reset. Since I haven't factory reset I find strange things are happening with my battery time (Seems shorter?) and Some apps like Whatsapp (the notifications locked up my phone twice and just now I couldn't run the app without it closing it self suddenly. I just restarted my phone to check my messages)
2) Even though games seem faster, the Launcher(s) seem slower. If I run an app, I don't know if it keeps killing the launchers because I watch it redraw them all the time. I've tried the stock TWlauncher for a day, ADW launcher for about half a day, and I'm running Launcher Pro right now. Launcher Pro seems the snappier of the 3 (less redrawing or less lag).
3) The text selection tools apparently messed up swype, so they disabled the double tap to highlight words. you need to be on a word and hit the swype icon/button to modify a word you already typed out. If you're not a swype user than it doesn't matter.
4) As someone else pointed out, they disabled opening up an individual SMS message in a larger black screen. There's no large view now for SMS messages.
Of course this is all without factory resetting. I have a lot of pictures on my phone and a fair number of apps. If you're not in a hurry to use 2.3.3 I don't really see a big need to switch to it. 2.2.1 was good (except for the database settings problem that someone mentioned in another thread)