I bought a 3G iPhone because I cracked the screen on my first iPhone.  I tried the 3G service and it did not work for me (dropped calls, etc). So I just use it on the EDGE network and it works fine for what I use it for.  There has been a lot of speculation on what the cause is, some rumors of a hardware issue with the iPhone, another idea was a software update (which Apple did a week or so ago, and I noticed no signal improvement on the 3G service), and now WIRED has an article about the quality of service might stem from the carrier’s infastructure.

Wired.com’s iPhone 3G Survey Reveals Network Weaknesses
By Brian X. Chen