>>2
I don't know where you heard that. CLISP is one of the suckier implementations. It has a few strong points, but I wouldn't use it except for those few special cases.
SBCL and ClozureCL are ``better''.
The term ``better'' is too subjective to actually mean much, but SBCL and ClozureCL both compile to native code, which CLISP doesn't, and SBCL is much faster. As far as library support goes, they(SBCL, CCL) also have more supported libraries.
As far as threading goes, SBCL and CCL have it, CLISP doesn't (unless you count green threads). There's also better debugging support, full MOP support and many implementation-specific stuff that I can't be bothered to mention. Also, CLISP's doesn't respect the ANSI CL standard in its full glory (unlike pretty much any other serious CL implementation), but I think they now have an -ansi switch.
In general, I'd say to stay away from CLISP and only use it when you need one of its more special features (a particularily nice one would be gmp support, another might be its interpreter mode).