Sidekick users and T-mobile users?

if I purchase a sidekick do i have to get the sidekick plan or can i just get a family plan?
cause i know on the iPhone you have to get and iPhone data plan

well you dont have to get it, but it nice having the sidekick plan because you get internet, email, aim, and text all unlimited for 20$. It is a pretty good deal. You dont have to get it though.