You should choose which directories you submit to, making sure they are relevant! Don't go for the sites that offer to submit you to 1,000 directories because the majority will be unrelated PR0s which make it much harder for your sigh to gain PR. It takes quite a bit of time to wade through and find the right directories, but once you've done it the rewards can be great!
According to me directory submission is like the listing of our website in Small search engine or directory. this may help to improve the website visibility on internet. and also useful for the website reputation.
Actually directory submission helps to create back link of website, and in seo it is very effective method. But, only directory cant rank up a website, seo has lots of method like : bookmarking in social media, web2.0, press release etc. In directories all free directories not working same, in this way sometime paid version directories works well.