Recs.
Updated
Specs
Pros
Pro Lots of production-ready libraries available
Common Lisp is arguably the most useful Lisp to "get things done". One of the main reasons of this is the large community behind it and the large number of libraries and frameworks already built to solve virtually any problem you may have when writing programs to be used by actual users.
Cons
Con Too many implementations
There are a lot of Common Lisp implementations out there (CLisp, SBCL, CCL, just to name a few). Some implementations are free and open source, others are proprietary and cost quite a bit. Though none of them follow 100% the Common Lisp standard for example, SBCL fails about 50 official ANSI Common Lisp conformance tests. Having so many implementations it may be hard to develop new libraries because you would have to test it for several different implementations.