WiMAX can be used as a mobile service, or as a last-mile service.
In the US, on the mobile end, Sprint is the farthest along, with xohm (as they call their WiMAX service, for reasons best known to themselves). I know nothing of any last-mile plans in the US; that seems to be more popular in Europe.

The reason a lot of people are excited is that it eliminates the need for a mobile phone with voice plan and data plan and tethering plan. (Depending on provider, of course.)

With a WiMAX tablet you can:

• Eliminate the phone
• Skip the voice plan (if you use VoIP, why pay for voice?)
• Not worry about whether to spend more for a tethering plan or risk getting caught tethering with no tethering plan
• Hope that your area eventually gets solid coverage
• Hope that prices (I don't believe any solid pricing data is available) don't go through the roof

Naturally, by the time the N810W is actually for sale, the coverage and pricing data will have firmed up, but some people are naturally optimistic now and perhaps more excited than circumstances warrant (right now).