I have more than a few friends with iPhones, and I owned the 3G and the 3GS myself before moving. AT&T had the following quirky problems in my area and still does.
-I'll call my friend on AT&T and it'll ring for a good 40 seconds or so. When I mention it, they claim their phone only rang once. When I had the iPhone I would sometimes test this calling from my work phone at the office and I was able to reproduce this problem more than once. I'm sure most people aren't patient enough to wait 40 seconds, unknown missed calls.
-I'll be relaxing at home or busy working, not doing anything phone related, as it'll just be sitting on my coffee table or laying on my desk at work. Bam, voicemail notification. No missed calls nor would I hear the phone ring.
-3G deadspots all over the place. Classroom? No 3G. Mall? No 3G. Coverage map online shows my area saturated in 3G. It's like AT&T only upgrades half of the towers to 3G so they can claim a market as 3G for their commercials.
-Delayed SMS messages and missing MMS messages. I decided to drop the iPhone the day a lady friend of mine sent me a "fun" MMS I apparently never got, and I missed out on visiting with her that night. Me not getting messages either SMS or MMS has happened more than once.
-Finally, dropped calls. I'd drop calls with a full 3G signal while sitting in my living room talking to my parents. That just flat out shouldn't happen, much less as often as it was happening.