With the exception of a couple major directories like DMOZ, Yahoo Direcotry, and JoeAnt I wouldn't even consider wasting my time. Maybe the occasional high quality niche directory, but honestly I would not waste time with 99% of the directories out there.
In my opinion I find them borderline worthless when it comes to SEO. Back in 2005 and before, sure they were fine, but I don't think Google takes most of them seriously anymore. Essentially, it's just another paid link.
In terms of traffic who on earth actually uses a directory anymore to find a website? ... Probably no one. I sure don't.
There are a couple exceptions depending on your niche. For law firms a site like AVVO is definitely worth your time given how much traffic it gets.