I started using MSN Adcenter a month ago and have had no real problems, other than they took more than a week to approve a few of my ads while others were approved in under an hour.

I like them since they have a lower opening bid (.05) than some of the others. It gives me an opportunity to drive super cheap traffic to my site. So even if conversion rates are slightly lower, my overall cost of doing business offsets that. In fact, I've noticed no difference in conversion rates, although I have to say that mine are probably higher than the average site because of the niches I target.

My sites are very new and I operate on a very thin profit margin. Without Adcenter, I'm not sure I'd even be turning a profit right now.