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?

>>25
Isn't that enough? They're all features that don't fundamentally provide anything that isn't accomplishable by a decent programmer via other means, and are known to be slow as fuck until someone proves otherwise. Just what else you need? Common Lisp isn't about ``ABSTRACT BULLSHITE'', it's about getting things done with proven 1994-era solutions. If you want to advance the state of lisp, look elsewhere.