I really feel that I should learn Lisp and there are plenty of good resources out there to help me do it.
I'm not put off by the complicated syntax, but where in "traditional commercial programming" would I find places it would make sense to use it instead of a procedural language.
Is there a commercial killer-app out there that's been written in Lisp ?
One of the main uses for Lisp is in Artificial Intelligence. A friend of mine at college took a graduate AI course and for his main project he wrote a "Lights Out" solver in Lisp. Multiple versions of his program utilized slightly different AI routines and testing on 40 or so computers yielded some pretty neat results (I wish it was online somewhere for me to link to, but I don't think it is).
Two semesters ago I used Scheme (a language based on Lisp) to write an interactive program that simulated Abbott and Costello's "Who's on First" routine. Input from the user was matched against some pretty complicated data structures (resembling maps in other languages, but much more flexible) to choose what an appropriate response would be. I also wrote a routine to solve a 3x3 slide puzzle (an algorithm which could easily be extended to larger slide puzzles).
In summary, learning Lisp (or Scheme) may not yield many practical applications beyond AI but it is an extremely valuable learning experience, as many others have stated. Programming in a functional language like Lisp will also help you think recursively (if you've had trouble with recursion in other languages, this could be a great help).