I've never had a problem with the iphone 3GS. (Except that I dropped it and broke the silent switch on the side). Occasionally in large gatherings I would have problems connecting to a network (and I mean LARGE gatherings - like during a Rockies game, when the couple of towers in that area couldn't handle all of the spectators).

Had 5 bars of full 3G coverage the entire way up Highway 85 on the western side of North Dakota with AT&T! And also full coverage watching the Leadville 100, so I could keep an eye on the live feed about the leaders while watching from where I was on the course.

But I'm still on the unlimited data plan so I wouldn't switch providers anyway.