well I like yahoo alot better then adwords, because they wont sove a stick up your ass, and secondly they tend to be alot more forward with you as a HUMAN reviews your stuff generally, and thridly no stupid quality score... But unfortunately Im still in love with google.
If you've perfected an adwords campaign. Setting up a campaign at yahoo an msn afterwards can give you an extra source of traffic, some say you don't as much for clicks there either because there's not as much competition.
I have tried them all and I prefer MSN Adcenter. I feel I get the best conversion at the lowest cost.