I like the butch harmon laser trainer the best. comes with a wonderful mat (don't pay attention to the ball flight laws on it) which doubles as a nice putting matt, the lasers work well (however won't fit w/o modification on midsize grips or larger) and all in all just the best i've used. I don't think they make them anymore so you might have to find them on close out or ebay.
The thing about flashlights is that it isn't a golf club. I like the lasers to be ON the golf shaft. So that way your feedback from swinging the club with the lasers on it can turn into normal feedback on your swing. If you just use a laser or just a flashlight you don't have that club in your hands. Plus it's hard to adjust for plane angle too. At least when you put the lasers on different clubs you can setup properly for each.
HOWEVER
The danger with lasers (happened to me long ago) is learning to trace a straight plane line WITHOUT having an on plane right shoulder. So be careful.
But as far as if they work, yes they do. I think the reason why i tend to hit the ball so straight with little movement is because the plane is the boss and i work on it constantly, even at home. If you are on plane, even with poor clubface control you know where the ball is going most of the time. Once you add some clubface control in there you can basically do anything you want with the ball.