Theres been some problems with sidekicks losing info lately. My DBF and I aren't fans of TMobile's lack of customer service, and phone service in general.
DBF had a sidekick, but after many problems with service, and losing his contacts and pictures not once, but twice (by no fault of ours!) he took the SIM card out of the sidekick and put it in the old phone he had before.
We will be losing TMobile as soon as the contract is up in April.
I'm sure others use TMobile just fine, but i wouldn't recomend them.