The main advantage using the Smith machine is that you can vary your stance(positioning of your feet). If you position them slightly ahead, there is more emphasis on the quads. It also eliminates lower back pressure when training heavy,which is a common problem with reg. squats. And of course, you can throw more weight on the Smith machine. It is also much safer in general..
Personally, I could only get a good workout doing reg. squats with a spotter. I don't trust myself training to failure...too dangerous !