Just my opinion....
I am with Sprint, and have been over 7 years and am very happy. I absolutely love the unlimited data.
Also while I hear a lot of people complain about slow data speeds, personally, my data speeds ate fine. I mean, yes, there are times when web surfing is a little slow, but it is not that frequent. Also, I seem to get good call quality too.
All in all I am satisfied with the quality of service I am getting with Sprint.