The coverage is debatable; sprint isn't the best, and not the worst. I've had several carriers over the past 17 years while working and living in Chicago (with periodic travelling). The worst was cingular: I was lucky to get through a call. Personally, I think the pricing is the best. Verizon comes close but it depends on the discount I would get.
I think conyers and argon hit the issue on the head. Nextel service itself is good, but the phones are horrible. I loved their service but I was going through a phone every few months. I switched to sprint because the phones are more compatible with my wife's hearing aids. Customers leaving for other devices is true for any carrier. I have several friends that did that several times, switching between several carriers and just recently to ATT for the iphone.
Sprint's customer service is the worst. I've always dreaded calling for any reason. I only stayed with nextel for so long because of my repair guy at the sprint store. Luckily I haven't had many problems since changing to sprint, but try solving an issue myself before calling. Since I've discovered the corporate number I rarely call the general 800 number. If they don't like it, too bad.
I think sprint has a host of problems that drives customers away, but as many problems as I've had they've never been severe enough to leave (although I've threatened to do so). It comes down to your own level of acceptable dissatisfaction.
Something else to factor into the lack of customer service is problems with the phone that customers attribute to the carrier's service, or think the carrier is responsible for. How many times have we seen a new BB owner have a "problem" and complained that customer service did nothing for him/her only to find that if he/she read the instructions they wouldn't have had to call in the first place... my 2 cents anyway.