Pros: 10 cents a minute when you buy the $100 card and the minutes don't expire as long as you but at least $10 per year after the first year. We got one phone via Costco for $30 and one from T-Mobile for $30.
Cons: A little ticky to understand if you are just dealing with a new phone in a cardbord box, but if you have the T-Mobile people open it up and set it up, it is all done for you, no extra charge.
Summary: Really great deal for older people or anyone on a budget. Kids can't exceed their minutes so it teaches them to be frugal. I had a traditional $60 a month Cingular plan and it cost me $750 a year for one phone. I still have all the phone contact I need but at a fraction of the cost. If you get others to sign up, the T-Mobile store will give you 200 or so free minutes. We've taken it to Vegas and all over southern California and it worked great.