Pros and cons. Switching away from a mainstream language is hard; you lose the community and all the work going to it. But you gain a lot of technical power, and possibly find a small community of really smart people.
Sometimes you can get the best of both worlds by doing something like ClojureScript, which is to compile to a mainstream language. That is compelling.
In the end, Lisp is really powerful, it's better than other languages, and it's worth learning. If you can deal with the lack of a huge community, you'll be fine. Clojure is gaining quite a large following, actually, so it's one of the best environments to learn Lisp.
Sometimes you can get the best of both worlds by doing something like ClojureScript, which is to compile to a mainstream language. That is compelling.
In the end, Lisp is really powerful, it's better than other languages, and it's worth learning. If you can deal with the lack of a huge community, you'll be fine. Clojure is gaining quite a large following, actually, so it's one of the best environments to learn Lisp.