There are some bottom lines that need to be kept in perspective here:
1. Forums are megaphones for the disenfranchised. People who complain use these forums to complain loudly. So you will ALWAYS see a preponderance of "complaints" in forums like this. It never fails.
2. Let's face it: Apple users are more fickle than any other userbase out there. If you go into any subforum here, you'd think Apple made the worst death trap products ever created. Yet they score tops in customer satisfaction and Apple can't seem to make them faster than they fly out of the stores. If iPhones and Apple products are crap, then the public sure can't get enough crap in their lives, can they?
This isn't to say that those complaining aren't valid in their complaints. But there are people quite happy, it's just not in people's nature to vocally express NOT having problems. And anyway, this place would be awfully silly if all it had were a bunch of "Geee, I love my iPhone, everything is great!" threads.
3. Shopping by forum is the worst way to shop. If you want an iPhone but are worried that the experience won't be good, then do what has been recommended for every phone, on just about every carrier: go get one and try it out for yourself. You have 30 days to try both phone and service on for size. Let your experience be your guide. If you have issues with coverage or don't like the phone for any other reason, return it and be done with it.
For the record: I've been happy with my iPhone since having the first gen, I've been happy with my 3G since July 11, and I'm happy today.
One last thing: don't fall into the exchange trap. A lot of people here get into this obsession about exchanging an iPhone 4 or 5 times. If in the off chance you end up with a defective phone, exchange it once. The second exchange should be fine. If it isn't, the seriously consider just getting a refund. Likely the store has a bad batch, and 4, 5, 6 exchanges won't make you any happier. Though if you want to try again in a couple months, go ahead.