A key advantage of Clearwire's WiMax network is a lack of bandwidth limits: As carriers boost speeds on mobile broadband networks, users will increasingly employ those networks like wired broadband--and run into the 5 GB limit beyond which insane per MB fees are charged--from 5 cents to 20 cents a megabyte, which is $50 to $200 per gigabyte. Clearwire sees its freedom from limits as an advantage, quite clearly.
Clearwire's CEO Bill Morrow said in a press release that the average 4G customer on the Clear network uses over 7 GB per month. That would cost a 3G user $100 to $400 extra, but it's included in a $45/mo unlimited 4G mobile plan (or cheaper for fixed home service); unlimited data is also included in all home plans (from $25 to $45/mo). A home and mobile bundle is $65/mo.
AT&T, T-Mobile, and Verizon will be under increasing pressure in markets that Clearwire enters and co-markets with Sprint (which can offer 3G back-up in the rest of the country) and cable operators like Comcast. I would suspect that if Clearwire's service tests out well, heavy users likely to be long-term subscribers and pay regular overages will simply move to a Sprint/Clearwire plan, using a 3G/4G combined modem or portable router.
On the issue of capacity, Clearwire said it is "doubling the number of transmitters and receivers per site, thereby boosting potential end user speeds by approximately 20-30 percent." This would answer a concern raised to me by Novarum's Ken Biba, who found in testing earlier this year that Clearwire wasn't dense enough in its deployments for its suggested download speeds. Clearwire also said it's increasing backhaul by 250 percent or more.
Update: Biba asked me (rightly) to clarify that adding gear and bandwidth at existing locations won't solve the speed issues he spotted, which require denser deployments (more nodes per square mile, for instance). Clearwire's bandwidth is all in 2.5 GHz, which needs relatively dense outdoor deployments even at the power levels Clearwire is allowed in that band.