They are indeed very quiet -- it sounds like a toy. The bbl is ported on this unit so that you can even use supersonic ammo in the gun. The ported bbl slows down the ammo to subsonic velocity and does this by bleeding off a ton of gas -- which makes the thing a pain in the butt to clean as nyeti indicated.
A much better solution, imho, is an MP5 with a muzzle mounted can -- like the Gemtech Raptor. A bit louder because of no ported bbl and you need to be careful about ammo selection if you want to stay subsonic, but you don't give up the veolcity that you do with the SD gun.
The cool factor is there and they are a ton of fun to shoot.