They are pretty good, depending on what you get. For some jobs, like putting up suspended ceiling, a self levelling laser that shoots a line quite wide (ie will hit most of a wall with a line) are really nice. For outside stuff, like figuring out just how tall the clothesline poll is going to have to be compared to the house, the type that shoot one beam in a line are nice. They usually are good to about 100 feet or so. The more you pay, the more features you get, and the better the accuracy is for longer distances (or rather, the better the laser - less spread over distance).
I have one, and I'm glad I do. It makes some jobs really easy (like figuring out slope on a wheelchair ramp.) But it won't make a good carpenter out of a bad one, and I can do just about everything I do with the laser with other tools. It's a matter of speed and convenience.