Had the 8900 for about 5 days now, had an 8320, 8310 and 8300 before it.
This is the best mobile device I have ever used. EVERYTHING about it is better than the 83xx in my opinion. With the updated OS even the battery life is much better. It's faster, brighter, crisper, much more professional looking and feeling. The keys are much more springy and responsive, the speakers are much higher quality and louder, the new browser is great and I even use it over Opera.
The only con I have is the lack of themes right now. However, I downloaded the newest version of the Plazmic software and made my own theme (which I have already posted on the Media section) and it was simple. I couldn't make any of the really intricate themes that some of the other users posted but I was very easily able to make a Today/Zen hybrid that showed my text messages in Today format and the rest of my icons in Zen format. Took about 30 minutes to figure it all out.
I can't put my 8900 down. I'm more excited about this phone than when I first picked up a Curve a couple years ago. It's basically everything great about the 83xx Curves but better in every aspect. I would highly recommend it.