I'm not sure - "simple" CLOS is really pretty simple and arguably works in a more intuitive way than most OO environments. However, if you do want more sophistication, as with most of Lisp, taking the Red Pill of the MOP can take you as deep as you want to go....
[Damn - I really need to do some more Lisp development]
I would agree with this... basic defgeneric/defmethod/defclass work is straightforward. It's when you start with the advice partial methods and the MOP that things get... exotic.
(I read the AMOP earlier this year and I'm pretty sure I haven't recovered from that mind bender).
