Picked this up a little while ago to go on my GSG-5L. Dialed it in today. In the world of red dots, it's nothing fancy but it's more than adequate for the GSG. Installation was cake. Two allen bolts (wrench is included if you don't have a couple sets of your own) and you're on your way. It took me about half a magazine to get it dialed in and I was punching out ragged groups for the rest of the afternoon. Build quality seems decent and solid, the adjustments went smoothly but there were good positive clicks between notches. I don't think it'll lose zero unless you toss it off a cliff. The knob to adjust brightness has the same tight, solid feel of any Bushnell, Primary Arms, or Aimpoint I've ever used. It's not quite as sharp and clear as any of those but, again, it was impressive for the price.
Bottom line - you can't go wrong with this red dot if you have a GSG. Just do it. You'll be glad you did.
Pros: Inexpensive, perfect fit, easy to use.
This review was written in the old system and had content requirements that are different than reviews written today.