I have used one of them. I liked it when I was using it, but it hurt my form in the long run. Problem I had was that you can anchor about 1" different and still move your head up or down enough to center the circles. My anchor creeped up over time and was affecting my accuracy. Went back to a peep, but the accuracy and vision I had with the no peep was pretty cool.
Don't know what they cost now (was $30 bucks when I bought mine), but I will sell you mine for $10 and pay the shipping if you want to try one cheap. Better someone use it than sit in my bow box. PM me if you are interested.