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ODP is also useful as a staring point for your link campaign combined with the Google directory which results from it, and is displayed under normal serps.
For the cost involved (zero), You would have to be mad not to apply to be listed.
Make a note of the date and category you submit to and if it's not listed after a couple of months, you can inquire about the submission at the resource zone (link in category Charter [webmasterworld.com]).
It's worth it for the PR from the listing plus the Google directory listing, and then there's the matter that by inferred definition ODP is an "expert" source of a link. That alone makes it worth it having, for any search engine that figures that into their algorithm.
To my mind that is the definitive answer.
It is free, but time is money. You can waste time
1. Getting onto DMOZ as it is unavailable quite a lot at the moment
2. You can waste time getting the "add url" feaure to work. There has been a lot of correspondence at WebmasterWorld about the problems
3. You can waste time chasing up whether or not it has been accepted/deleted/lost.
In the end, if it does not get in, or takes the proverbial 2 years to be found in a dusty corner by a DMOZ editor, then your serps ranking on Google will not be substantially altered on way or the other.