I was a Sprint customer for 8 years, then I had a billing problem and they were rude, lied to me, and didn't do what they said they would do (I had to call back twice and they still didn't credit my account, then shut off service.) It was the collections agent I had to deal with to get service restored, that finally issued the credit they'd promised me and apologized.
Then when I went to cancel my service, even though I had been told it would only be $50 per line, they charged me $250 to cancel it because one of the lines (the one I'd had for 8 years) was grandfathered in to not qualify for a reduction in the fees as you near your end-of-contract-date. Way to treat your long-time customer, Sprint.
Now, I has AT&T cuz I has iPhone. Other people say it's crappy, but in KC, we have no call dropping issues of any kind and the signal strength is actually so much better than Sprint's (which is SO ironic because Sprint's World HQ is about 5 miles from my house).