The user interface within the advertising console clearly frustrates most search marketers. They say it takes them at least twice as long to manage a campaign in adCenter than it does with Google AdWords or Yahoo Search Marketing. The page to page load times are slow, the features are sometimes found in obscure locations and the campaign management is not as intuitive, according to some SEMs.
But yet, many SEMs love the ROI they see from Microsoft adCenter and Live Search. Some say they have the best ROI because they currently do not have "low quality affiliates" messing with their inventory. Both Google and Yahoo have large publisher networks, which many advertisers find to mess with their conversions and quality. Microsoft has yet to open their publisher network, aka ContentAds, to the public. They are testing the ContentAds program with internal Microsoft sites and selected and trusted publishers. It is because of that, impressions will be lower than at Google and Yahoo, but some SEMs say the quality is much higher.
Forum discussion at WebmasterWorld.