MS is currently beta testing their PPC platform for publishers.. once it goes public I would imagine that AdCenter users would get more traffic since their ads would have the option to be displayed across the publisher-network (just like Adsense)
I've been using MSN for a week now - if you are doing bulk uploads - forget it. They cap the number of adgroups/campaign to 500. And, you have to initiate each adgroup before the campaign starts. Sad, but true.
