I have one on my .22LR pistol. It's a blast! I think for fun value, not much approaches it for the money involved. It came on the gun, but I don't think I'll take it off. If I don't want to use it, I just leave it off and use the sights.
I have noticed it take longer to acquire the target. This has been mentioned a few times, so at least I don't feel too stupid about that.
They also work great if you are using a night vision device that would otherwise prevent you from using the sights. You could probably deal with some serious nocturnal rodents with a cheap russian monocular and a laser. Not exactly a combat rig, but cheap enough.
Ross