If you mean as a publisher (not an advertiser), time after time, my tests have shown that YPn performs horribly. They can't deliver a targeted ad 2x in a row, they have very low ad-inventory, and EPc is very low.
if u r talking for front page of yahoo for adv. its good otherwise google adword program is good compare to yahoo.
Currently they are accepting only US users .. so if it fits you,apply.. but I recommend Adsense.. it is flexible and has better options..
Yes, yahoo website is very good for advertisement. The advertisement on yahoo is cheap than other famous site. Yahoo also provides toolkit. Shaddi.com and matrimonial site, Naukri site are very dedicated advertiser of yahoo site. They got lots of hits for their site from yahoo portal.