Whenever I need logging in my Common Lisp applications, I use Nick Levine’s cl-log library. For many applications, spending the time to run CL:FORMAT during runtime is unreasonable. It is often much better to log something in a raw form and do any formatting/presentation offline rather than online. The cl-log library is the only one that I know of which supports binary logging readily.
Additionally, Nick is incredibly responsive. I found an oversight in a corner-case of cl-log last Friday at 1am. I emailed him about it at 1:19am. In under 5 hours, I had email back pointing me to a new release with that case corrected.
I hope to do a more complete comparison of the existing log libraries at some point. At this point, know that I’ve tried all of the ones on the Cliki which had obvious documentation or examples, and I’ll be using cl-log.