I remember when it first arrived on the scene, many indies said, 'Who needs it? I can do my own festivals!' type thing. I know, I was one of them. Since then their site has grown and now I'm an advocate. There's lots more added with so many festivals it would take days to go through them all. Their filters are reasonably good when you're looking for specific genres, cut-off dates, up and coming etc.
Another bonus and interesting fact, at time of writing unless they change this in the future, is if you're trying to list your short film on Imdb.com as a new title (always hard to do if you have no or little track record) is Imdb seem to automatically place it up as a title, even though your short may even get rejected from that particular festival you entered (shush! trade secret ;) - go figure. Imdb own Withoutabox so their systems may be a bit tied in there as it seems an almost 'automatic' inclusion, once their editors see it coming from Withoutabox.
(cross-posted to shortfilms)