We use ucw for http://paragent.com (click on "DEMO" to see our app in action live). We actually use the older _dev branch of ucw, as we started over a year ago before the _ajax branch had really settled down. In fact, we've ended up removing much of the AJAX from the site for design reasons, and gone to a simpler page model with the latest site update. ucw has been pretty reliable. OTOH, we don't get hammered on the front end that much, so we may not be really stressing it. Most of our heavy-lifting is all on the back-end connections to the agents, where we have to maintain thousands of ssl connections. We have been very happy with our decision to go with Lisp.
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.
