I've been on Straight Talk for several months with my iPhone. No problems at all.
The biggest issue with prepaid carriers is that you are kind of on your own. You can't call them about every single little issue you may have, like you could Verizon or AT&T. So, I would hesitate a bit to suggest Straight Talk to older folks who need a brick and mortar store when they can't figure something out with their phone.
That said, I never had problems with Straight Talk CS. I think its the best deal around... You get AT&T or T-Mobile unlimited service for $45 flat (albeit no roaming).