Very good point. 10 cents per minute is a very reasonable overage charge (think VZW charged $.49). I've found that 90% I'm away from WiFi I only stream Pandora or GPS turn by turn so even 3G speeds should suffice. I've also read people saying their data wasn't throttled WAY past the stated mark. I guess that would depend on location and other variables. So do you need to keep buying the $30 plan cards or whatever or only once? I was under the impression you keep buying them, but I'm assuming if you can go over your minutes and just be charged the 10 cents that your phone doesn't just quit working. Obviously I'm a prepaid noob!
With the prepaid plans, you have to refill your amount each month (or set it up for auto fill). As far as going over your minutes, they will shut your call services down if you exceed the max and have no extra money on your plan. What most do is log in to your tmo account (on line) and add something like $20 to it. The if you go over your minutes, you don't get turned off, it just draws from your excess funds.
As far as the voip goes, it works for me because I don't talk on the phone much, and when I do, I don't like it, so I get of quickly. But if you like your world perfect, voip is not yet there. There are plenty of times when you have a few second time delay as well as times of stutter and static.