This is an interesting device that I hoped would give me a good ab workout. And it gives me a workout, but it feels more like my arms and back are doing the labor. Which is not a bad thing, because I can sure use it.
It comes with a tiny little mat to support your knees, but seriously, get a yoga mat for this thing, you're gonna need it.
For what it does, it does it well. Smooth glide, and I really have to put some effort into it. I would not recommend this for people who have back problems at all, but I find it pretty easy to use. No instructions necessary.
For added benefit, there are many places you can find instructions for variations of ab roller exercises. After I began exploring some of them, I got better results with this machine.
A fair price for what you get. But I'm not sure it's really working my abs. YMMV.