I just switched over from Verizon and so far (about one week of service), it's been great. I work in Manhattan and in a tall building so the 3G network is a little slow but at my apartment in Queens, it's great.
No dropped calls yet but I don't talk on the phone that much.
They did make an upgrade to the network in NYC and the surrounding boroughs recently. I really have nothing to compare it too since I'm new