FWIW, I had Straight Talk with a Samsung Galaxy Proclaim (S720C) on VZW's network for 8 or 9 months. Service was pretty good, for the most part. But here's the rub with most (if not all) prepay services: no roaming... or limited roaming. You have to go to postpaid plans to get full service coverage. At least, that was my experience.
Also, from my perspective, when you get up to 4 lines or more of service, the MVNO plans begin to lose their value, especially if roaming is not included. My service with ST/VZW worked on VZW native towers only. If I went to an area where VZW roamed, I had no service.
As usual, YMMV.