I've used them for squats and benching. They are safer for guys that don't have a spotter (training partner)
Frankly I think you can gain alot from it. While it doesn't completely work the stabilizer muscles as much, you can use more weight in a safer manner with the Smith machine.
Case in point, I bench more with the Smith. Part of that is because it is easier to lift since you aren't expending the energy in balancing. But even more so, I can comfortably put on more weight because of the added safety factor.
I recommend it, but I don't recommend it at the gym to people, because I like the fact that I'm one of the few that uses it!