My only complaint is that it only works locally, meaning there's no way to fit it into my workflow on projects that involve testing on remote servers. (Admittedly, watching for filesystem changes via SSH/FTP is non-trivial.)
I cannot say enough good things about CodeKit (which is basically a more intense version of LESS.app, from the same guy). It's improved my workflow considerably and saves the hassle of concatenating and minimizing server-side when I'm cranking out projects right and left.
(and it also supports Sass, Compass, Haml, CoffeeScript)