I have it on my G19 and love it. I don't use it so much for eye level shooting, but doing a draw and shoot from the waist or around cover where you don't want to expose much of your body/head; you can just see the laser dot and shoot your target as long as it is sighted in.
I use it as a just another tool like you would a tac light, etc. Obviously a novice shooter needs to be profiecient with their firearm/sights/etc before adding a laser..
+1 for G19C, I just added one of the CT laser grips to my FIL's G27, he was having trouble focusing on the front sight. He is 70 so I thought that this would help him out some. As for target shooting I don't think it would help out as much as in SD shooting. It is a great tool to practice dry fire and trigger reset with.