From Ingvar Matsson's Nov. 8, 2007 blog I learned about the Project Euler and started working on the problems. There are 176 problems this week and new problems are added every one or two weeks. The site is so popular that the host service provider cut off database access last weekend that forced the author eliminate more dynamic features.
I decided to use Common Lisp as the programming language of choice, though otherwise I've been using Objective Caml more often in the last five years.
So far I've solved 74 out of 176 problems (42%). I use SBCL and SLIME and I'd say Common Lisp and SBCL in particular is quite efficient at the task. Most of the solved problems take no more than a few seconds to finish at the interactive Read-Evaluate-Print-L oop environment.
I chose Common Lisp over Objective Caml mainly because Objective Caml does not provide automatic bignum support, which renders number crunching cumbersome.