Most of the iPhone users i've talked to say its been just fine on AT&T. I've got an iPhone 4 and it's working great, 3G speeds are plenty fast, and calls haven't dropped at all. Voice sounds bad, and AT&T might be worse in that respect than other carriers, but poor call quality seems to be part of a trend that followed the switch to digital phones, they just sound worse than the older analog ones and definitely worse than landlines.
Most of the AT&T hate seems to be coming from users in large cities like SF and NY. I expect that the signaling issues that TechCrunch and others have reported on that are specific to AT&Ts network are hitting hardest there.