A friend asked me to review my switch after 3+ years on AT&T to Verizon with the iPhone 4s. I've been using the Verizon phone for nearly 3 months and I spent all summer sitting next to an intern using the first Verizon 4 iPhone, so I got to use them side by side before switching. Here are my thoughts after a few months of usage:
In general I always have coverage everywhere I've been in Portland and in cities beyond, more so than when I had an AT&T phone in the area. I haven't noticed the reported call sound quality increases (cell phones just generally sound like talking into a tin can and still do) but I do have to say I've never dropped a call since I left AT&T. When I would be driving on the freeway in Portland, my calls on AT&T (through bluetooth, of course) would drop about half the time. It was almost like clockwork when I'd hit the city limits of Portland that my calls would begin to drop on AT&T. That has never happened with Verizon. In nearly three months, I've never gotten a "network busy" error either, which is something that happened a couple times a month on my old AT&T phone.
The only downside I've found is that data is a bit slower. Not super slow and you get used to it really fast and forget about it, but I remember thinking web pages I always accessed on AT&T were loading on Verizon in a few more seconds than usual. I do tons of stuf online with my phone so this can be a dealbreaker for people working 100% online that require the fastest data connection, but I gave up a slight bump in speed for reliable networking. The Verizon phone always seems to have a 3G connection.
There was one other aspect aside that also made me switch and it was the increased coverage that Verizon has over everyone else. I noticed in the coverage maps that Verizon extends a mile or two into the wilderness more than AT&T and every single time I went for a mountain bike (MTB) ride in Oregon (Bend, Dallas, McMinnville, McKenzie River, Mt St Helens, Hagg Lake) my AT&T connection would disappear either in the parking lot or the trailhead. I've taken a couple MTB rides since I switched and on one ride I always had a connection whenever I stopped to check and another I had a connection about 3/4 of the stops (only in a small valley of dense forest was there no connection). This was really important to me since I broke my wrist last year — I know I'm no longer invincible and if anyone riding with me ever crashed on a MTB ride, normally we'd all be screwed and have to hike out with an injured person. Having some chance of phone service made me feel a bit safer whenever I go off road.
That's about it, increased coverage, calls never drop, but data speeds are a tad slower. The costs are about the same but I have liked switching to Verizon since the local store has a very competent staff that helped me get phones on launch day and have fixed a couple of tiny issues since I got the phones (like changing callerID, etc). The local AT&T store was mostly unhelpful in the three years I would sporadically interact with them.