As a blogger, I can tell you that decent blog directories can be few and far between. There is a resident listing of them on a few blog forums, but a lot of them are generated by companies with nonsense blogs. If you're a blogger who wants to blog about life, you don't want to be on a directory with a bunch of bologna with ebay links all over it.
I guess the question we have to ask too is are there many directories that people use to find blogs or do they exist just for the link value? As you will see with most website directories actually humans never use 99.999% of them to find websites.
Nod, and it's not always a good idea to link up with something with a poor ranking. But have you considered offering more than just a blog directory: perhaps tools for bloggers to use, forums, and goodies. And be very careful not to allow it to be a free for all, or it will be a total mess of spam.