Is SLIME any better than Geiser? I've been using it with Racket and it's pretty good.

http://www.nongnu.org/geiser/

Geiser is great, but SLIME is much better mostly because Common Lisp was designed with interactive programming in mind(the condition system, the object system, dynamic variables ect. are all features that help with interactive programming). You get an almost smalltaskesque feeling with slime.

As the linked doc says, geiser draws inspiration from Slime, but of course it's for Scheme rather than Lisp (or Common Lisp.)

From what I've read in the past I think most attempts to use Slime with Scheme haven't been wholly successful, hence geiser.