About using ipones on t-mobile what's the deal?

I really want an iphone but I can't get a contract with att so I'm now with tmobile I have a blackberry 8520 its really cool but in my my mind the iphone is better now everybody tells me that I can get one with tmobile but some say it will work fine some say things won't work can I use app store an all the apps I have a unld web plan can I use that for the iphone
What all I can't do on it
What's the deal?