Add me. I had a iPhone, a 3gs, and now a 4 on T-mobile and haven't been on a contract in years. I love having un-limited everything. Sometimes I wonder if Edge sucks, but mostly I am pretty happy. The 4 is a factory unlocked one. I hold out hope that the iPhone will come to T-mobile soon and will be able to use the full capabilities then.
My (ex)husband has had an iphone and 3gs on T-mobile too at my urging. He switched from Verizon for them and seems sort of annoyed that they have the 4 now, but must be forgetting all the time between during which he enjoyed his Apple phones.
I went to Canada for my 4 in February just as Verizon was offering them, and I had to consider carefully going or switching. :S Still not sure what was better. Now it seems they'll unlock phones here at full price too, I missed out on white (either way,) and paid more, but...
Best phone to have on tmobile prepaid. Hands down. Paid $150 for my 3GS on craigslist, jb'd it then unlocked it on iPad BB. Would like GPS, but I don't travel far and mostly use wifi. Spend about $100 a year on prepaid and have an iPhone I can impress people with.