Been one for nearly a year.
I like it and have no problems. I'm the same way, don't want contracts especially in todays job market.
No complaints other then the price being $5/mo higher then having a contract. But TMO's always been willing to bend over backwards for any problems.
I.E. When I first signed up I got put on the BIS plan vs BES and they switched it without charge! Even changed my # without charge when I ported my wife's phone onto the plan. (Good to know: the area code on all phones within the plan must match). I think I spent 3 hours working with the tech because the previous ones had made such a mess of my plan.
Anywho... Flexpay does what it's supposed to and let's you bring your own and not have a contract.