VZW is saturating their network. AT&T has come back a great deal. A lot of flak comes from their terrible network at the iPhone 3G launch. Now I feel it's just a lot of exaggeration.
I have had the opposite experience here in Fort Collins, Colorado. When the iPhone 3GS launched we didn't have 3G locally except in some areas on the south side of the city, but calls were reliable and things were good - except slow on the data front. Since then, they rolled out 3G and things were even better, but over the last year the call quality has gone down, and number of dropped calls has gone way up. I get a dropped call about 1 out of every 6 calls, or worse, and the longer the call the worse my odds. If I'm not dropped then often the call quality will degrade to the point where I have to call them back. I generally have all bars and the "3G" icon on my iPhone. Calling my wife - also on AT&T - is even worse because odds are good one of us will get dropped. It's been really awful since about the end of Feb.
Probably this is just a couple of towers in the area, but my experience is the opposite - things were good and have gotten much worse in the last 9 months.
Maybe some people are complaining to complain, but in my case, I'm honestly contemplating going back to T-Mobile. The more I get dropped, the more irritated that I get. If I call AT&T, they either note it and tell me they are working on it, or tell me to reset my network settings... which doesn't help. At this point, I'm thinking about just paying off the ETF and giving up... I'll take EDGE on T-Mobile (and a lower monthly bill) over 3G on AT&T and dropped calls all the time.