I have Sprint networking via VirginMobile on an iPhone 4S. It is slow, however, I mostly use my iPhone 4S at home or work via WiFi, and when out and about, the 3G Sprint Data is "Good Enough" for my needs. I accept this because having paid up front for my iPhone 4S, I now only pay $30/month for unlimited data (300 mins voice, but I almost never call anyone).
As hexonxonx says, Sprint 3G is slow. If you have Sprint LTE in your area, then an iPhone 5 should perform very will. If no Sprint LTE in your area, then an iPhone 5 would have the same Sprint 3G data.
NOTE: If your iPhone displays 3G it is moderately good speed (not great, but it could be worse). If it has the little degree º symbol (a very circle), then you are getting 1xRTT (CDMA), which is so slow you could walk to the web site faster. If you are mostly getting the 1xRTT degree º symbol, then you are not going to be happy with your Sprint service.
At least where I am, I mostly get 3G and like I said, when not in range of my WiFi, it is sufficient. The few times I've gotten 1xRTT, I have found it best to think of other ways to get the information I want.