Basically this is a rear peep sight, a different way of targeting using the a-5. This replaces the classic MP5 stock sight on the a-5. I had a problem using this becuase im use to the standard MP5 iron sights, i trained using them and am very use and comfortable with them.
The Peep sight i guess is an all right upgrade if your looking to burn cash, but i say save yoru cash and upgrade to a RIS flatline barrel, or R5 hopper, expansion chamber or something else before you buy this. You might like the sight but i couldnt stand the bloody thing and it caused great eye fatigue.
If you have the money to blow go for it, but i would reccomend this being one of the last objects you buy to upgrade your gun.