Leg extensions can be a pretty dangerous exercise. I've seen a lot of information on how they actually reduce the activity of the VMO (vastus medialis oblique) and reduce the range of motion of which it activates. This is all I could find right now,
http://www.t-nation.com/readTopic.do?id=1306675 .
The type of leg extensions that are talked about in that pdf are often called terminal leg extension or terminal knee extensions. They can be done in quite a few ways.
I would much rather do squats, lunges, TKEs or step ups over leg extensions.