I'm using them right now. They actually aren't a prepaid carrier so not sure if you will get a better response in the "Other US Carriers" forum. Here's my take-away:
- Genuine AT&T SIM. AT&T logo and everything.
- Not throttled
- Uses the standard AT&T APN settings
- Mobile hotspot is enabled
- There is no overage. Data is not throttled once you meet your limit. It simply stops.
- Excellent pricing all the way around, and unlimited SMS, MMS, domestic calling
- There is no international roaming option available, period.
- If you hit your data cap, you can change your data cap plan for that month at any time without penalty.
- Uses the full AT&T post-paid network including roaming partners unlike prepaid MVNO's such as Cricket
You just have to decide whether you prefer the cap-and-stop versus others which have cap-and-slow (Cricket) or T-Mobile's free streaming from certain providers. For my needs where I go to a lot of rural areas and need the domestic roaming and hotspot feature, it's spot-on.
Edited to add:
My only concern is will they stay in business. I'm guessing they are really, really small. I called customer service and she was very helpful, but I could tell she was answering the phone from home and had a baby crying in the background.