I've just spent a week using the T-Mobile Sidekick II, and I think it's probably one of the best mobile devices I've ever used. And I can say, with total conviction, that I will never, ever purchase one and neither should you. In fact, after the experiences I've had today with T-Mobile, I'm not sure I can ever recommend one of their products again.
Wow! I was actually excited about this device, but I am excited no longer; I wonder how many others are like me? Note to Danger (manufacturer of the Sidekick and Sidekick II): you may want to source your device to a different service provider.