Sometimes with directory submissions the benefit is not so much in the direct traffic you get but in the fact that it gives you backlinks from what are seen as 'Authority Sites' in the eyes of Google. That value would be hard to measure... I can't think of a way. I'm not a super pro tester and tracker type though
I do know I had one brand new blog, as an experiment I submitted it to lots of blog directories. Most of them required me to have a link back to the blog directory - so it wasn't even a one-way link (the best kind). In a really short period of time my site was a PR5. Two other blogs that I set up at the same time, and had about the same number of articles/posts on were only listed as PR4 and PR3.
I actually removed all the links to the blog directories and scrapped the idea of doing that again when Google came out with a
PR update. It really made me sit back and think, the only thing I did different with these 3 blogs was the blog directory submissions for the PR5 blog. So I think it makes a difference.
Of course web directories usually give you true one-way backlinks, so in theory they should be even more valuable to increasing your page rank than the blog directories that I submitted to.