I got my iPhone last October. I used to talk trash on iPhone, but now I don't know what I would do without it!
I think voice plans might vary in price from region to region, I am not sure about that. I have the unlimited plan, which is an extra $30 per month on top of the voice plan. That won't be available any more, the top data plan will be $25 per month. This will have a cap of 2gb per month, with a buy-up option for heavy data users. I use the internet and such on mine all the time, but the data usage stats showing on my phone are about 12 gb. If that is lifetime, then I guess I average a bit under 2gb per month.
There are not many wifi hotspots in my city, but if they are available where you are, that will reduce the data traffic on your plan. (If you have wifi at home, you can switch to that when you're home, too.)
I love all the apps, and carrying the internet around with me. The thing I like the least is the battery life, but I'm hoping OS 4.0 will help that a bit. Oh, and not being able to view websites that use Flash, but I think a lot of websites are under pressure to reduce their Flash usage due to complaints from iPhone and iPad users.
I personally haven't had much trouble with AT&T's service, (other than data being slow sometimes), and I've been with them since 2001 (they were Cingular then). I used to get a lot of dropped calls when my husband had Verizon and I had AT&T, but I don't think we've had one since he joined my plan.
as much as I love my iPhone, if you're interested in smart phones I'd suggest also checking out some of the phones out now that run the Droid platform. You'll still have to get a data plan, but there might be more flexibility to compare prices between providers.
Also, you should ask people where you live how they feel about the providers they use: I think coverage can vary from region to region. I am in Kansas City, home to Sprint. Most people here HATE Sprint, as their prices here are very high if you don't get a discount through your job, and coverage is very very bad here. On the other hand, my best friend in San Francisco has Sprint, they have excellent coverage and more competitive rates out there.
I know several other iPhone users are on RO, hopefully they'll chime in, too.