I'd like to add a little to this old topic... I am highly impressed with MS's Adcenter too. Their customer service is excellent. I submitted a question and within the same day, they actually called me to explain the answer. I've used their phone support before and find it to be courteous and helpful, even for a very very small advertiser like me.

I also like the low .05 bid on keywords. You don't get that on Yahoo or Google. I'm pretty good at finding keywords that are overlooked and can bid .05 or .06 and get a lot of impression and clicks for very little $$.

Just my two cents.