I gave up this past Friday. I canceled my order with AT&T, went to an Apple store and bought a 6 plus there. After all that waiting it only took about 20 minutes to get the phone from Apple.
With the "death grip" on my iPhone 4, I can get it as low as one bar at times, but not past that. I've never lost service or dropped a call. My 3GS does exactly the same thing.
I have wi-fi at home and work, so I've left it on all the time the past several months. Haven't noticed any difference in battery life at all. And I've been thinking about dropping to the $15.00 data plan since I'm not using as much 3G.