I use mine as a promotional tool and a traffic generator. In that way it is hard to tell how much money it is really generating. It also adds to my PR - and you can never have enough of that.
I'm about to start a directory myself, mostly for my own websites. Will see how it goes.
I never administrated a directory before. Any advices you might want to share ?
Keep on top of submissions, the amount of spam you'll get will drown out the genuine submissions. Install captcha, and make sure you set up the directory so that people making a submission actually have to visit your site, not just use automated submission software.
Isn't DMOZ still the largest directory in the world, with 4,603,789 sites listed?