The only thing I don't think is good for professional users (at least creatives) is the monitor. At least the last time I looked into them they weren't the best compared to having a decent external monitor.
Or for those who already have a high-end external (matte) screen and don't want to pay for the lack of hardware expansion and an lcd we don't need.
Also I know it's apples fault for the 3GB in the Quad MP but how is it reasonable to compare the i7 iMac with 4GB vs MP with 3GB?