I'm from NYC and I switched from Verizon (our gold standard here).
Here are my observations:
-Outside performance- same as Verizon to me, I've only had a few calls outside that have dropped or have had choppy reception... since the 3GS came out. Inside is a different story. Depending on where I am... if I have to make an important phone call, I know to use a landline or go outside. Certain areas of my office and my apt, the call fails 8/10 times. There are dead zones. For me, when I'm out and about it's not an issue. Call failed is something I got used to inside. It is rare when I get that message outside.
-Data network is usually good- I use the GPS/maps program all the time and there are only a handful of times where I had to wait on the street due to a bad connection
-3G v. edge. I keep my phone on 3G all the time and for the most part it stays on 3G. For the majority of the time, the iPhone stays on 3G. Again, it's inside, like in my apt. where 2-3 times a week, the iPhone will stubbornly stay with edge for a bit.
-Wait or buy now. I'm like you- always wanted it but nobody knows for certain what will happen in the future. The 3GS rocks and the way I see it, if it indeed does go on to Verizon, I'm hoping the mass exodus will reduce the strain on the 3G network. I bought my 3GS when it first came out so really it's just an extra year of AT&T if the iPhone goes to other carriers.
Overall, I'm happy with making the choice to switch and if I could do it all over again, I'd use AT&T. It can be a pain at times but for the most part it makes calls < accesses the internet.