We are using a slightly modified version of sbcl. Because of the large number of persistent connections we have to handle, we can run into issues with FD numbers and the default serve-event implementation. We have swapped in our own epoll-based serve-event to get around this. Otherwise, sbcl has worked very well for us.

Most of the sbcl hackers hang out in #lisp on irc.freenode.net, so the level of help available around the clock is amazing. There have been a couple times when we were pulling our hair out, got into the #lisp channel, and had a solution in a matter of minutes. Highly recommended, and a very smart group of people.