I'm on T-Mobile and I upgraded my 8100 to the released version of 4.5 (Vodafone) to be able to make my new Jabra BT8040 work (mostly). I've been running it for about a week and I like it, but it's a trade-off right now. There are bugs. The much-discussed camera white-balance problem is the worst (IMHO). The browser is better, but there is a bug in text entry boxes (if you move the cursor out of the box, you can't get it back in to enter text again). The sound volume in the media player and on the phone is too low (I have to keep mine turned up just about full volume, and the media player isn't loud enough for me to listen to MP3s while riding my bike). I'm sure there are a few other issues.
On the other hand, the HTML email is very nice. Much more valuable than I had expected. The Docs-2-Go is a real plus. I really like being able to download native MS Office documents from email. I like being able to listen to media through my A2DP headset, although I wish Jabra and RIM would fix the incompatibility many are now suffering with. I have not had the slow-down problems reported by some (I've upgraded my OS a couple of times, and it is always slow for the first few hours for some reason, but then returns to normal).
All and all, I'm keeping 4.5, while hoping the T-Mobile release will come soon and fix some of these issues.