The last time I used it, SLIME integration for Clojure was very slick. It was a while back so there were a fair number of moving parts to align, but I'd guess that has gotten better.
It is worth doing, especially if you've never used another integrated Lisp environment, but I wouldn't recommend laying it on top of learning Clojure. I'd say learn Clojure first, then try out emacs and the emacs integration.
Slime and Clojure have improved, mainly through clojure-jack-in and ritz. Of course the Common Lisp Slime experience is still better but the essentials are definitely there with Clojure.
There are several heavyweights in the Clojure community that use VIM and have developed good plugins, and Rich never seems to be using SLIME during presentations, so you can definitely get by fine without it.
I agree. Emacs is a power tool for writing Clojure. I learned Clojure first with tools I knew, then learned Emacs later. For me, that was a much better way to deal with the cognitive load.
It is worth doing, especially if you've never used another integrated Lisp environment, but I wouldn't recommend laying it on top of learning Clojure. I'd say learn Clojure first, then try out emacs and the emacs integration.