For those not familiar with Emacs, IntelliJ IDEA with La Clojure is also an okay choice. I use IDEA for Python, Java and Scala; and still switch to Emacs when I code in Clojure. The reason is that Emacs has really strong toolset for LISP, where many of the features are missing from other environments, or implemented partially.

Slime/swank (nrepl replaces it, but I'm just slowly transitioning to it), provides code completion, documentation lookup, REPL, macro expansion and even live code updates on a running application; which can be a bit of a pain to setup on other editors. And the integration in Emacs is really great, as the repl buffer is really just a text buffer, the same as the one you are coding in.

But the biggest reason for me to code LISP in Emacs is paredit. It really makes all the trouble with balancing parenthesis and moving around the sexps really easy. It might seem counter intuitive and a bit troublesome at first, but I really recommend you to try it. For a short and enthusiastic video on paredit, see http://www.youtube.com/watch?v=D6h5dFyyUX0 -- and maybe the other Emacs Rocks! videos to learn more about modern Emacs.