I was a Sprint customer for 6 years, and I thought they were fantastic. I never had a problem getting phones replaced that were defective, their corporate store employees were extremely friendly and easy to deal with, and the times I did have to call I ended up getting great deals to stick with them.
Plus, their service -- especially in Ohio -- was tremendous. I rarely had full bars, but I never was without service and I had maybe 2 or 3 dropped calls in the entire time I was with them (compared to AT&T, where I drop a call at least 5 times per week).
I'd still be on Sprint if it wasn't for the iPhone.