Common Lisp interpreter

I'm reading Land of Lisp and the guy uses CLISP (and CLISP specific extensions), so I'm using it for now. But I've heard good things about SBCL and others. Which one would you recommend? Should I stick with CLISP?

>>20
I don't think it's why SBCL has an interpreter, it's probably just for completeness-sake, not like writing one is that difficult.