According to my experience, yahoo search do not generate much traffic compare to google. I will say about 1/10 compare to google search
Yahoo could give different depends on specific niche. I have seen some niche, especially related to Japanese gadget stuff, have a good traffics from Yahoo over Google. This is because Yahoo is more prefer by Japanese over Google for now.
People use it but the traffic is much less than with Google. That being said if you have a high traffic niche then you can still make a lot if you can optimize for Yahoo.
Yahoo can bring some good traffic .To get top on yahoo is more time consuming compared to google , but it is still worth in my opinion .
i get about 30% of my traffic from yahoo- this is alot for me.. yahoo search users aren't that techy that they click ads.
I think Yahoo is the second search engine where people goes to check what they are looking for online.. So you cant totally ignore yahoo and get focus on simply Google. Yahoo too has many traffics but moreover if you does your site optimization in right way you will definitely will see good results from it.