As much as I'm happy with my T-Mobile experience I'm not sure I'd call them nice. According to Consumer Reports Verizon is the clear leader in customer service overall, with T-Mobile and AT&T not that different for most people.

What separates T-Mobile is that they're much less evil. Things like making pre-paid cards easy, giving you a discount if you're not on a plan, not going to bat in Congress for SOPA etc. They may be stupid or unfeeling sometimes, but they never seem to be actively opposed to one's freedom like the other big carriers are.