I have also had no problems or issues with the 8900, either the phone itselt or T-Mobile.
I too have the 8320 and although I enjoyed owning the phone and it's operation was very smooth, the 8900 does feel firmer, keys are easier to locate, the screen is MUCH brighter and it is an overall an excellent improvement over the 8320 and a GREAT addition to the T-Mobile Blackberry line!
T-Mobile Customer Service has always been first rate with me and they have always been not only very helpful but very accomodating as well!
I lost my 8900 recently and it would have cost me $359.99 ($429.99- $70 loyalty rebate-still not a bad deal!) with a 1 year commitment. I have a kid's connect line on my account and although that line is not authorized to be upgraded to the 8900, the CSR spent some time working the system so I could get another 8900 for $349 ($10 less is still a good deal!). Unfortunately, the $100 rebate is not applicable since I don't have data service on that line.
I ordered the phone on the 18th and it will be delivered by UPS Monday, the 22nd. No backorder and 4 day delivery time is really pretty good!
I can't say anything bad about either the 8900 or T-Mobile.