We use Sprint because my other half gets a 30% discount from where he works. I have no complaints over the service, but the styles and types of phones they carry chaps me every now and then.
When we switched, I got a Palm Pre (an original) and I really liked the Palm phones, when it came time to upgrade, the Pre2 had just come out, but Sprint decided to NOT carry it! So, I stayed with my original phone until last year, a few weeks before Christmas. I had a minor graphics flaw that they fixed in store, but in doing so, destroyed the earpiece! The replacement that was sent (a factory refurb) had the touch screen not working and the replacement for that one had a non-working keyboard. A fourth (or fifth, I forget at this point) had about half the keyboard sticking and not working a portion of the time, but it got me through Christmas.
We went in February (January was a busy time for us) and I upgraded to a Windows phone. I like the HTC Arrive, but it is a lot bulkier than other company's versions of Windows phones...if I had waited another month, I could have gotten an Android that is much thinner!