There’s basic game theory in play here. Posited: You want tethering.
Case 1: Spend $20 once to buy MyWi
You tether your device. It either continues to work indefinitely (my personal experience) and you pay no extra cash per month or they detect it for some reason and force you to pay $20 a month starting the next month. Total cost after one year ranges between $20 and $240.
Case 2: Just sign up for the personal hotspot function from AT&T to start with
You tether your device and begin paying $20 a month extra continually and the total cost after one year is definitely $240.
Seems like a nobrainer to me, and as I mentioned, I’ve been doing this for 8 months.