Some how I got lucky and had my MySpace Resource Site listed in the Yahoo Directory for free. But the traffic I get from it is terrible - usually only a couple of visits daily. I have seen my SE Rankings within Yahoo increase as a result of my listing though, and now my link is in the other worldwide Yahoo directories as well. I probably wouldn't pay for a link in the directory, but since I got in for free I might as well keep it in there, right? $299 can get a lot of backlinks on other sites and may end up being more effective than paying it only for a "chance" to get into the Yahoo Directory.
It does help for your Google ranking, and when you think about it, it's less than $1 a day. However, you can obtain the same results by submitting to a bunch of sites with high PR.
Submitting to Yahoo directory will unlikely directly increase your traffic by very much. It will however increase your webrank, which means that you likely see some better positions in Yahoo (also Google & MSN)
It's definately worth it in my opinion. You can't get a more trusted link other than DMOZ in Google's eyes. On top of that, it will give you a significant bump in the Yahoo SERPS. I submit all of my sites to the yahoo dir. The benefit I get definitely outweighs the cost.
For me: It was worth it. I had a new site that did not have enough Back Links to get indexed, until I paid yahoo. Would I do it for all sites, No..