Depending on where you live, Sprint's coverage is terrible. You can check out their coverage map at their website or in a store.
I don't work for them, so don't quote me, but I have heard that they do slow your data service down as well. I know that they are supposed to be switching their data packages soon to limited amounts and that they will not be grandfathering their customers to the unlimited packages.
Use the WiFi on your iPhone when it's available. You'll find that it's just about everywhere. You may see a difference on your actually usage as well.