The sites I own that are properly indexed by Yahoo get a decent amount of traffic, but it's getting in there that seems to be the biggest hurdle nowadays. For example, one site gets about 40% of its search traffic from Google, and 35% from Yahoo. Interestingly, the number of keywords the site receives Yahoo search traffic from is twice the number of keywords used in Google searches, so there's a much bigger variety of words and phrases that the site's pages are returned for. As for some other sites, they only get their homepage crawled every now and then and don't rank for anything. It's hit and miss with Yahoo.