I've used it numerous times; I think whether you do it or not depends on how competitive your niche is. If you are battling to be at the top of something extremely competitive, then everything helps, especially a link from a site with as much authority as yahoo.com
i remember owner of this blog added one of my links to yahoo dir for free because i linked to his blog . i never found how he did it
I think it is worth it since it would get your site easily indexed by most search engine. It would be a nice investment. I will advice you to o for it if you have an extra money but don't waste your money if you need it to pay some bills in your house because it would take a lot of time before your site gets a good number of traffic from search engines.