My iPhone 3G gets coverage in most areas, but yes, poor in buildings depending on the building. At my gf's house I get No Service messages about 85% of the time, the rest of the time I get a single bar on Edge.
At my house, luckily, I get decent coverage even in the basement (where it only goes to No Service maybe 1% of the time).
The voice quality over 3G is pretty much the same as Verizon's, in other words about the same quality as a regular house phone. Have had a couple of dropped calls in random spots but I am not sure if that was me or the other caller since they were cell phone to cell phone calls.
Overall, AT&T is not that bad at all. I'd give them a 4 out of 5. Verizon is superior though, at least in my area and if the iPhone ever went there I'd even be willing to pay an extra $10 a month for it.