No personal experience, but check out this post by our friends over at RV Mobile Internet Resource Center:

AT&T Unleashes Affordable Unlimited Data for “Connected Cars” – Chevy OnStar or ZTE Mobley.

We've been using our AT&T Mobley for the past month or so, and it does everything we expected it to do. We've seen good 4G/LTE speeds everywhere we've been with it so far, even well after we passed the 22GB mark. After the 22GB, our unlimited Mobley plan is subject to "deprioritization" (not "throttling") where our speed can slow down when on a congested tower until the congestion clears up. If that has occurred, we haven't been aware of it. I'm told the temporary speed cut could drop us down to mid-3G speeds (1.25Mbps), so it's possible we may not have noticed if we weren't doing anything heavy at the time. The key factor though, is that unlike throttling that cuts your speed for the balance of the billing cycle, congestion based deprioritization only drops your speed for the period of congestion. At $20/month, we're pleased enough with how well the Mobley has performed that we dropped our $100/mo Verizon 20GB 4G/LTE plan.