For phone calls, I'm finding the GS4 to be excellent. Much better than anticipated, coming from the GN2.
From a data perspective, I did this speedtest at my BIL's house yesterday. They have VZW service (I'm newly on AT&T myself, coming from VZW) and where they have always had weak signals causing dropped and sketchy calls in their home, my calls were strong and clear. And I had 4/5 bars of strength too.
I've never seen data speeds that fast before. It blew away their wifi network speed, just below the LTE results.
Naturally, at my house, I only have HSPA speeds. But the phone calls are good. Just goes to show that every carrier has strong and weak spots in their coverage.