Sorry I'm a bit late to the party, but I'm currently using a fat pixel back (Sinarback 54M, 2004 vintage, I believe) with the Kodak 22-megapixel sensor in my portrait studio in West Orlando. It's hitched to a Hasselblad 553ELX, mostly shooting the 80mm/F2.8 CF Planar, the 120mm/F4.0 CFi Makro-Planar, and the 150mm/F4.0 CF Sonnar. And if I'm shooting tight head shots of someone interesting, I use the 180mm/F4.0 CFi Sonnar (a truly stunning lens). I'm very happy with the resolution, and the skin tones--most of my clients are black, because of my location, and it does a far, far better job of getting the skin tones right than my Nikon D810, which tends to make black people very orange-y without some rather strenuous correction.
If you'd like to see what it can do, here are a few full-resolution samples I've got stashed on my website. These have had a bit of Nik Sharpener applied to them, but it's just building on what's there.
http://www.presquevu.com/x8505.jpg (I *think* this was shot with the 180 Sonnar and an extension tube.)
http://www.presquevu.com/x2783.jpg (Shot with the 120 Makro-Planar--the same lens Platon uses, with his style of lighting.)
http://www.presquevu.com/jeremiah.jpg (Shot with the 150mm Sonnar, wide open, with extension tube.)
http://www.presquevu.com/macro.jpg (Shot with the 120mm Makro-Planar and the Auto Bellows; this was a test shooting a coffee sack I have hung up on my wall. The area of the image is approximately 1.5 x 1.2 inches.)
http://www.presquevu.com/apa071.jpg (This was actually shot with my first Sinarback 54M and the Mamiya RZ67 plus 180mm W-N lens; I had to sell the camera and the back to help fund my move back to the US. As soon as I could find--and afford--another Sinarback on my return, I snapped it up, but I use it on a Hasselblad here.)
Hope this conveys just what this old fat-pixel back can do in terms of portraiture. It's "only" 22 megapixels--but it's 22 *glorious* megapixels!