I have an 'unlimited' data package with AT&T, and they do the same thing. I think I can only use 10 Gigglebytes of hotspot data before they shut me down, which if you really think about it, sounds an awful lot like a limit. They also slow my data down to really unusable speeds when I get to 22 gigs of mobile data. Now, I'm no rocket surgeon, but dammit, that also seems an awful lot like a limit to me.
It really, really, pisses me off. I'm on the move 6 months out of the year, and that data plan is my only internet connection when I'm travelling. It's really easy to use up 22 gigs a month, and it's super frustrating to have to budget my 'unlimited' data allowance, especially considering that it costs me well over 200 dollars a month.
Sorry for the rant. Back to the OP, is this plan and mobley device really unlimited data, or do they throttle you and cut you off? I'll buy one right now if it's truly unlimited.