I used one for years and loved it. Where we live, there was only one mobile company we could use and it was too expensive at the time, so I got a tracfone. I was able to get service absolutely everywhere with it (even in the boonies of WV), BUT when they went GSM, it wasn't the same.
The minutes are expensive, but you get a lot of coupons for free minutes and you can find coupons on the internet. Like the other poster mentioned, if you call from your tracfone you get billed for service, but just don't call from your cell phone -- call from your home phone.
Now my DD has one. They have a great special going on where you buy a phone card for $19 and you get a reconditioned phone for free.