I'm on T-Mobile, and I use the TM506 to tether. I'm in Dallas.

Max BT bandwidth is 700 kbps. I always max that out when connected to the 3g network here. However, when I tether via cable to my laptop, I get 1.6 mbps.

On edge, I would get anywhere from 128 kbps to 210 kbps, and it was super laggy.

NOTE, there is a firmware update available for the TM506. DO NOT INSTALL IT. It disables BT tethering. (it also improves GPS lock time and a few other things.)

If you buy a TM506 and it has tethering disabled, you can flash an old version of the firmware over it to get tethering back. See the T-Mobile/Sony Ericsson forum on howardforums.com