As Ausie-guy said, there' s been many threads out there.
I' ve used both the Hind Sight & the No-Peep, and definately recommend the No-Peep. A bit of a pain to get it " just" right. But you can always just fine tune it as you go along. I doubt I' ll ever shoot without one ever again.
If you have any questions or I can be of any help, just send PM me. Be glad to help.
John