I had one for a year and a half. I liked it, but upgraded to a t-mobile plan right before a business trip about a year ago.
The flexibility is great, especially if you only want the phone 'just for emergencies'. My DH picked up a pair since I have a 45 mile one way commute and got a flat one night. I was nowhere near a phone, and had to walk a couple of miles to one!
They have ways for you to earn free minutes. You earn free airtime when you get someone else to sign up. When you register online you will earn more minutes. You can also purchase more minutes online, and they throw in free minutes when you do that. One time, they inadvertently charged me twice for an online purchase and the way they corrected it was that they gave me the minutes purchased for free! They really are willing to fix issues, I'd had to call them a couple of times and found them easy to work with.
I had one of the analog phones, and had one drop out spot in that trip...but I'm sure that if I'd had a digital phone, I would not have had any problems. Also, the phones show you exactly how many units you have left and when they expire.
We took them to WDW, and the roaming was supposed to be 4 units/minute, but we were charged 2 per minute. This was handy when I had to take my younger son to the hospital at 2 in the morning! I was calling with hourly updates (double ear infection after swimming that day-poor guy).
DH liked that he only had to buy a 10 minute card every month, but I ended up purchasing 60 minutes every other month. I started to price the various monthly plans, and t-mobile came into my mall and had a great deal, so I got it. If I hadn't needed to be accessible on the business trip and hadn't gotten a decent plan, I probably would have stuck with the trac phone much longer.
It's a great service.
Suzanne