I have read good things about AT&T pre-paid. You might also want to look at Aio wireless which is AT&T's pre-paid company. A little less money but does have some limitations. HSPA capped at 4mbs and LTE capped at 8mbs. If those speeds are okay with you then you can save a couple bucks.
Uses AT&T's network, current data rate is capped at 8Mbps for LTE devices and 4Mpbs for non-LTE devices (like the Nexus 4). Coverage excellent; building penetration excellent. No roaming that I can tell, but probably not necessary anyway.
I have been on the $55/month plan for a few days and all seems well.
Been on ATT Go Phone prepaid for a couple months now. Love it. No problems at all.
2GB Data/mo Limit. You can buy more data if needed at $10/1GB.
Love it and great coverage.
What do you need? My mother has a Go Phone plan on some sort of Samsung Galaxy. It's great for her because she gets voice calls and texts fairly cheaply, with no surprise costs. She uses it on wifi at home and other places, and because Go Phone won't do per-MB for smart phones, she can't accidentally rack up $30 in data charges.