I have t-mobile pre-paid. You get a choice of three basic phones, nothing fancy, not even pics. But you get to keep unused minutes each month that you renew. BUT, after six months, (or 7 or whatever it is), you only have to renew ANNUALLY. That means you pay the lowest fee you want for however minutes you need, and you don't have to renew again for one year. I have about 300 minutes in the bank and I won't have to pay again until September. It's great if you don't talk alot and don't need any fancy bells and whistles.
I too have T-Mobile prepaid, and it's the best that I've found. As hughb mentioned, you only have to renew once a year, so as long as you have minutes left, you can just by a $10 card and be set for another year. When my wife and I switched, we bought the $100 card (which gets you 15% additional minutes on all subsequent card purchase, I believe), and then just buy cards as we need them, which extend our coverage for another year each time. Jon