Pros: Generous expiration times, coverage
I originally started T-Mobile prepaid because I was on Verizon and wanted to play with some unlocked GSM phones. I didn't want to start another contract at thetime, so instead I started a prepaid account with T-Mobile.
I found the terms to be very fair... minutes expire after 90 days, unless you get to the point where you have purchased $100 worth of airtime. At that point you hit the "Gold Rewards" level and minutes do not expire for a year. Up until expiration if you add any airtime the unused minutes renew to the next expiration time. I have a couple of thousand minutes loaded on the account, and every 11 months or so I throw five bucks on to keep the account alive.
I have found T-Mobile's prepaid network coverage to be pretty good. My house is almost a Verizon dead spot, and T-Mobile works pretty well in areas of the house where Verizon does not.
As long as the coverage maps (which I have found to be pretty accurate) on T-Mobile's Web site show that you have coverage where you need it, I can wholeheartedly recommend T-Mobile for prepaid service.