Sprint was really bad 10 years ago (horrible service, horrible phones) but they've gotten very good in recent years. I just got a Sprint Motorola XPRT and the whole process was incredibly smooth. Called customer service and got my plan upgraded in 5 minutes, went to the Sprint store 30 minutes later and swapped phones. The rep there filled out the rebate for me and transferred all of my numbers from my old phone.
However, unless Sprint has exclusivity, it's committing suicide by buying such a huge pile of iPhones.