I've been eligible for an upgrade with Sprint since December 2011 but I have one year left in NJ and then I'm not sure where I'll end up so I might have to switch carriers depending on coverage.
I've been very happy with Sprint's voice service. Absolutely no issues other than a couple dead spots at work (where I only pass through on the way to the cafeteria so it's not a problem). Their data service is absolutely horrible in my area. For the first six months after I first moved to NJ in 2010, I used to be able to stream music while driving but now it so frequently keeps buffering and cutting out that it's unusable.
Honestly, if I didn't spend 90% of my time on WiFi at work and at home, I'd have switched to AT&T or Verizon a long time ago. The only thing preventing that really is that they no longer provide unlimited data. On the flip side of that argument, what good is unlimited data on Sprint if it's so slow that I'm not even going to hit the other carriers' 2 GB cap anyway? As I said, the majority of my time is on wireless data and I use less than 300 megabytes per month with Sprint.
For now, I'll be sticking with my Evo 4G. It serves its purpose even though I get $500 per year from my residency program that I could use towards upgrading my phone right now. I'd rather put that money towards something else since I really have no faith in Sprint's ability to deliver on their promises. I do have WiMax in my area but I never use it because of the fact that its penetration into buildings is about 5 inches.