I've had the 8900 for about two weeks now. It replaced my 8320, which I had for 2 years.
LOVE the screen. I debated between this and the T-Mobile MyTouch 3G and the 8900. 8900 has a higher quality screen and a qwerty kb, and I thought I'd give the cellular networks more time to build up their 3G infrastructure, so I went with the 8900.
the picture (as in camera-taking) quality is noticeably better on the 8900 over the 8320, but definitely slower in taking pic's. But I also had a lot of blurry, low quality pic's taken on the 8320 and the 8900 makes up for its slowness by giving a more consistently in-focus shot.
The folder/app icons take some getting used to. they aren't quite as obvious in what the icon represents as previous models.
Its also supposed to have a faster processor, but I think its actually slower than the 8320 (but not by much). And I don't have a bunch of crazy app's installed. Internet email and Gmail are the only things constantly running.
Major complaint is the keyboard (almost enough to make me consider going back to my 8320). I would swear the width is fractionally smaller on the 8900, but the spec's show both being 60mm. I have fairly small hands/fingers, but typing is noticeably and annoyingly more difficult on the 8900. The model I tested in the store had more severe angular keys with a little ridge at the tip, which were fabulous to type on. These are more smooth and flat and I'm not enjoying composing emails on this thing (and it is primarily a business phone, so I do a lot of typing).
RIM also put the external audio jack (on right side of phone) above the volume rocker, so I find it more difficult to adjust volume by touch and not sight.
and, of course, they changed the cable format to microUSB, so I had to buy new car chargers and spare USB charger cables.
Vibrate mode is much more subtle, which I don't like. If I set the phone on my desk and a phone call comes in, I can't hear the vibrations as I could on the 8320.
ah yes (geez, I keep thinking of more nagatives), and the speaker is terrible. Using speakerphone is an ear-straining exercise to decipher what the other person said; the quality is atrocious. I always thought the 8320 speaker was pretty good.
but I get more talk/standby time with the standard battery, a moderate improvement to camera quality and a sexy new phone, so I'll probably stick with it.
Hope this helps.