While I have admittedly limited experience with it, CFFI worked awesome for the couple things I tried to do with it. I don't have any CL code in production yet, but enjoy using it when I'm doing some recreational programming, or looking for something that's easy to embed in a C program (Lua's nice, but it's trivial to take the ECL Common Lisp implementation and hook up emacs-slime to it, even when it's embedded in a C program)