I like it. It'll eventually find it's home on an M4, but I can see it being used on just about any gun. (A Sig 552 came to mind as I was typing this. Don't know why.)
Build construction is pretty good. For $70 (or whatever it was, can't remember), it could have been a little better, but I'm not picky. I'm pretty well pleased by it. The uber-modular-ness of it makes up for that.
Finish on the metal is decent. It's a little rough on the edges, and there are seam lines on the plastic.
Overall, I'd give it a 7.5/10. Maybe an 8. Haven't used it on the field yet, so I'll update accord