They have 2 additional plans for AT&T customers. One is $10/month for mobile share plans and the other is $20/month for unlimited with potential throttling after 22GB when network is busy. The $25/month unlimited plan is for non-AT&T customers and that one doesn't seem to come with a throttling fine print either.
Funny how that works and it's still $15/month.. I don't think another bill a month is really worth it for anyone who has some form of hotspot functionality on their phone plan already.. For example, T-Mobile offers unlimited Hotspot on your phone to all customers basically up to 10gb a month before throttling kicks in.. It does go down to normal 3g speeds after 10gb though and it's pretty unbearable.. For 15/month however I guess it depends on how much car-traveling data you'd like to use on your other devices..
If I didn't have a second "Work" phone with unlimited hotspot, i'd probably jump on this.