I would personally recommend that you guys don't use ridiculous amounts of data, for it will surely throw up red flags at AT&T. I would stay away from torrents, streaming large video files, etc...
I have been using it for a couple days so far, and it works great. speedtest.net gives me 1.6mb down, and 314kb up. I am forced to use it since I am visiting my parents in Flint, MI and they don't have internet, or any nearby wifi spots, and I absolutely needed to get work done on the iMac. If AT&T would offer a tethering plan, I would pay for it, but seeing as they don't, I am forced to do this.
I have run into a couple problems over the past couple days though. First, occasionally it takes several tries to get the phone to connect to my iMac. Secondly, after using PDAnet for a while (45 minutes or so) I either get the recharge beep (that you get when you plug the phone into a charger), or the phone simply does a hard reset. The battery still shows good charge, so it isn't the battery draining (plus I keep it plugged into the wall charger while tethering), and the phone is not very warm either (unlike when I use GPS for long periods of time where it gets really hot).
I dont know what is causing the reset problem, but it seems to pop up when on a graphic intensive site, or a site with streaming videos (liveleak, youtube, etc)