I fell in love with Lisp at University, but I've not really used it since. It was a wonderful affair; it messed up my head for a while, but it made me a better programmer and gave me a lasting respect for the power of Lisp.
I had a data structures class, and an instructor I didn't respect. She said we could use any language, so I learned Lisp and used it for all my assignments, because I didn't want her to be able to criticise me. I was kind of a dick back then; still am in some ways.
I remember the final project was something implementing red-black trees and doing the rotations and balancing of them. My program was less than two hundred lines long, and a thing of beauty (I'd figured out Lisp pretty well by then). All the other implementations were several thousands of lines of much clumsier code.