It's a little device that creates a wifi signal where ever you are. For example, Virgin Mobile offers the device and daily or monthly pre-paid plans.
Virgin Mobile | Broadband2Go 3G/4G Wireless Internet Plans
Yeah, if you have an unlimited plan with AT&T, more than likely, you've been grandfathered in. In order to use the tethering, you'd have to chose a new data plan and you can't switch back to the old unlimited plan.
I use the data plan on my phone more than I use the talk/text minutes. I am on the internet all the time. Plus, I look at a lot of Netflix & Youtube, & I stream music, too.
I need to crunch the numbers though, and shop around for the best deal. It could be that I end up paying about the same either way, lol.