I have them on both my Glocks, and had them on the 1911 and J-frame I used before I went to the Dark Side. I find them very effective when used in realistic - as opposed to square range - defensive shooting. They are especially useful when shooting on the move and in low light.
If you haven't used them in dynamic shooting (box drills, snakes, scenarios, etc.), you really can't grasp how well they work. Just standing there on a square range and shooting with lasers doesn't really tell you anything about them, or any other kind of defensive shooting for that matter. It's too bad they are illegal in IDPA; that might be a good proving ground for them.
I see the arguments made here against lasers are similar to those made against "flash sight picture" defensive shooting. The laser, like the sights, are not used to "aim" the gun; rather, they are used to confirm an alignment that the practiced body has already made. When engaging targets at high speed while moving, the laser gives the shooter instant feedback about how his movement is affecting the gun's alignment.