Cost will be the main advantage compared to using cellular network. WiMax will be also faster but that depends on the number and the usage pattern of the users. (The way they are billed will have an influence, too.)
I also like the fact that the WiMax service will not be tied to cellular service providers. And I expect (hope?) the WiMax will be more open and will be easier to implement in various devices and O/Ss.
And about the availability in US, I don't have much informatin but according to wikipedia, 'In September 2005, Samsung Electronics signed a deal with Sprint Nextel Corporation to provide equipment for a WiBro trial(
Link1 Link 2) '.