I've been shooting with a No-Peep for 2 years now and I like it. As previously stated, you have a wide open view, great visibility in low light, and works as well as a peep for me (accuracy wise).
They seem to be a love it or hate it deal though, some love 'em, some hate 'em, some just look at it and say "what the heck is that doo-hicky?".
One caveat though, they are a pain in the butt to set up the first time, but I think its worth it. Helps to have a friend make the adjustments while you remain at full draw.