I am not happy with any cell phone provider right now... been through a few, and had bad things associated with all of them. I am with Cingular right now... fantastic plan, but the quality has been horrible lately.
I have used AT&T Wireless and have nothing at all good to say about them. I was charged for all the calls that were never answered... if I dialed it and it rang once, they charged me.
I have used Sprint in the past. My bills tended to be higher with them, but the quality was always better than with any other company I have used. The only thing with them is, I know they recently merged with Nextel. When I worked for the security company, we used Nextel, before I finally talked the company into switching. I handled all the bills, in additon to everything else, and I found billing errors on every single statement. Before I came to work there, they were just sending the checks... I found major problems, and it was very hard to get them resolved.