I've used T-Mobile for the last 4 years. Generally pretty happy with them - their customer service is unobtrusive and generally pretty good, website is solid, plans are reasonable. I think I did accept a 1-year contract when I upgraded to unlimited voice + data + texting, but the initial plan wasn't contract-based.

Coverage in the Bay Area is great. Out east it tends to be a bit spottier than most of the other networks - in rural areas or inside certain malls my relatives on AT&T or Verizon might get service but I won't. They're generally fine in major urban areas and around major highways, but somewhat sparser on the long tail.