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I have used Sprint's network before, and it can be pretty slow. You will never see those 30-40Mbps speeds that are theoretically possible. I agree with the article Christopher quoted; you will probably experience about 3Mbps download speeds on Spint WiMAX. It's good enough for most use, and you can stream Netflix or music with no problems. If you travel much, though, you will see Spint's real weakness - super slow 3G. I would regularly experience 3G download speeds in the 250-350Kbps range, which is barely useful.
I haven't used WiMAX -- and hopefully someone who has will weigh in -- but I found this interesting article about WiMAX vs. LTE:
"The version of WiMAX Sprint uses should deliver 30-40Mbps download speeds, and LTE can deliver up to 100Mbps. In the real world, though, they achieve nothing like those speeds. That's because a network's performance depends much more on how it's built than on the specs of the underlying technology.
"Our Fastest Mobile Networks results tell the tale. In our tests across the country, Sprint's WiMAX averaged about 3Mbps down, and it's currently capped to 1.5Mbps up. That's slower than most home broadband. Verizon's LTE network tripled that download speed to 9.46Mbps average, although uploads averaged about 1.35Mbps. That's roughly as fast as many home broadband packages."