Various providers have various levels of service in different areas. Here in AZ after 7 years with Verizon I have not had a single dropped call I can recall. On other providers (work phones) it has been a much rougher road.
Regarding "bars" on the phone. I can tell you that the phone getting a strong radio carrier from the tower DOES NOT guarantee that the switching / control system has the ability to properly process / route all the incoming data, which can easily overload / confuse a poorly designed / overwhelmed system. Carrier strength and system integrity are different issues.
All said, I believe ATT has the WORST rating in terms of service nationwide, and Verizon is the best. Again, this is the average rating from NATIONWIDE surveys. Your area of the country might be the exact opposite of this. It all depends on where you live.
While I love the iphone, I just am not willing to risk it with ATT. Hopefully the phone will be unlocked in the next version.