My mistake - it wasn't an F-15, but an A-6. 
http://www.gallagher.com/ejection_seat/
still pretty terrifying though.

The confusion comes from a story I read around the same time (this was a couple of years ago I first came across it) about a guy who ejected from an F-15 at supersonic speed and survived, and then went back into piloting them.
http://www.ejectionsite.com/insaddle/insaddle.htm