I have just gotten started with medium format and I have been doing some testing with my system against my canon. Here are two shots, one with my Canon 5D Mark III (50mm lens) and one with my Phase One 645AF with a Leaf Aptus-II 6 back (80mm lens). The shutter speed was set to 60, stop was 2.8 and ISO was 200 on both.
Canon 5D Mark III:
From what I can see, the Canon does a lot of auto corrections to the color and white balance while the Phase one is more true to the actual environment. The Leaf back is only processing 4 more MP and lots of detail but is incredibly slow.
More reviews to come!