I don't see why it's not legit. I've seen that kind of offer before for Sidekick. The 300-50 is the original offer from T-mobile and I've seen many other places offering better deal.

Honestly, I regret buying the Sidekick. It's slow as hell and not convinient to dial phone calls. I basically only used it for the first 2 months and it's been sitting in my drawer.