4G is in the very, very early stages, like mobile phones when they were huge suitcase-size devices.
With that in mind, Clear's options are simply not very compelling. They charge a lot of money for a very tiny coverage area. Clear charges more depending on whether you buy a "home" plan or "mobile" plan. The "mobile" plan is significantly more expensive. But the "home" plan is not guaranteed to work indoors, even though they give you equipment that requires AC power. What a scam.
Sprint's plan is better; their 3G network is everywhere and is included with the 4G plans. (Or rather, 4G is a no-cost add-on for their 3G plans.) This is more compelling because Sprint 3G is everywhere, and their network is pretty solid. I get much faster speeds on Sprint's 3G network compared to what I got on T-Mobile or AT&T. (And like I said, when I do get a good 4G signal, it beats my 6M/768k DSL. In fact, with a bad signal, I usually get something like 1.5M/1.5M, which still beats my DSL in one direction.)
As the wireless carriers move from 3G to 4G to take load off the 3G network, I think 4G will become as popular as 3G. But relying on early-adopters paying $40 for worthless coverage is not a great business model. And relying on people to use wireless broadband when the fine print advises, "not guaranteed to work inside buildings" is also stupid. Why pay more money for a slower connection that's not guaranteed to work like copper?
That's why nobody is in the Clear store. Anyone with $60 month to spend on wireless broadband just uses Sprint, and everyone else just tethers their phone because they only use wireless broadband once a year.