I use a t-mobile prepay (with a 212 area code ) in the states

Top up, and your minutes dont expire.
When you have put more than $100 top up, your minutes wont expire for a year even with no further top ups.
You can also access your account online and top up from there with no problems.

T-mobile is also useful as it is the only US network with UMA.... which can be useful for doing, um, some stuff that T-mobile dont want you to do

The only thing that REALLY hacks me off about US cell phone networks is that the recipient also gets charged to receive calls or texts...coming from the UK that is really alien and annoying, and also a bloody rip-off.
Other than that, very happy with T-mobile