I'm just coming to the end of my contract with T-Mobile, and have had a Viewty with their web n walk package - unlimited browsing basically. I've never hit the 'fair use' policy and constantly run my laptop through it whilst away from home with no problems at all - over a year now (originally on a U900).
The only downside I have noticed is that it automatically downloads low res images in every page - they can be manually replaced with the proper ones (hover over then shift + R) but this is probably something I missed in the settings.
Torrents won't work, but Skype and Live Messenger do.
Sppeds are good enough to get by on, but doni't even think about playing an FPS online through one!