I would rather have you spend time writing quality posts on your blog than submitting to directories. Quality content will make visitors bookmark your site and keep them coming back. Sure directories will increase your link count, but the amount of traffic you get from them is typically small.
I'd have to agree, I haven't seen a whole lot of activity coming from blog directories. However I have done some backlink work on different related sites that involve topics I may discuss. I've also noticed that twitter is helpful because you get to give a subject and the link.
Submit your blog to blog directories, blog commenting and RSS Feeds helps. Engaging on other techniques such as article submissions, classified ads, social bookmarking, press release and forum posting helps.
Directory submission does take a lot of time, there is not too much affect, however it is still helpfull. I usually just use semi-automated free service at http://www.easysubmits.com and their list is more than enough for me... :)