It's good T-Mobile is allowing people out of their contracts. After all, how can you claim a service contract is valid when you failed to provide any service?
However, this wasn't T-Mobile's fault past the fact that they trusted Danger (and Microsoft) with the management of the data servers. I would stick to T-Mobile, but definitely not with the Sidekick. Try a G1, MyTouch, or even a BlackBerry, and avoid the cloud.
That said, even Google and RIM could screw-up, and cause a loss of data due to a failed synch or something similar, no modern technology is safe from idiocy. Always keep your own backups (if possible) of anything you consider valuable.