Like mentioned above, Sprint depends a lot on where you live. They don't have 4G LTE rolled out nearly as extensively as the other carriers yet, but they do have 4G WiMAX coverage all over the place for their previous-gen phones. with my Galaxy S2(WiMAX) I had 4G coverage in most every major city(100K+ people) that I traveled to around the midwest, it was usually pretty easy to find a 4G signal. Now that I have a Galaxy S4, getting a LTE signal is a little more troublesome since their LTE network isn't anywhere near complete yet. When I can get LTE, it's damn fast, but I probably end up on 3G more often than not now. 3G on Sprint is still fine for streaming music(I use google music and Pandora a lot), playing games, etc, it's just going to be damn slow for streaming large videos or downloading big apps/programs.
But yeah, on Sprints 3G, you're probably going to be lucky to get 1mbps download speeds, its usually more like .5mbps, which is fine for a mobile webpage, or even music, but you're going to slow to a crawl trying to get any video over that consistently.