This is a post mainly for the gear heads on here. I'm not a professional reviewer or a blogger for the matter, but more of a "writing from experience" writer. I've been shooting with a digital dslr since the Canon D30 was released back in 2000. Wow, 14 years has past since then! Time flies when I'm having fun, lol. Anyway, dslrs have matured tremendously since then and pretty much any dslr you buy today completely destroys anything released back in 2000.
Up until this year, the last medium format camera I used was the Pentax 645 and the Pentax 67. Man oh man did I love using the Pentax 67 with Fuji Astia film. Digital medium format cameras were always the price of cars ($10,000-50,000+) which I could personally never justify. Well, this year I was blessed with the opportunity to purchase a used mint Pentax 645D. I paid $6000 for mine early this year which is pretty much around the price of a high end dslr. Since the new release of the Pentax 645z, you can get a new 645D for $5000 as of July 2014. Yes, $5000! for a new mf dslr. A lot of money for most folks but we are talking about a camera that debuted for $10,000 a few years ago.
I had been shooting with a Nikon D800e prior to buying the 645D and I must say it was a phenomenal dslr for portrait work, which is what I shoot 99% of the time. I sold it less than two months after I received my 645D. I just didn't feel the need to keep an expensive dslr camera as a backup so I bought a cheap Fuji S3 pro to fit the bill instead. Although the Nikon D800e and the Pentax 645D are very close in resolution (36 vs 40mp), the output was enough for me to make the decision not to keep my Nikon. Just like how medium format film was bigger and better image quality wise, medium format digital brings the same to the table. The 645D and the Nikon D800e were released close enough to contain similar image technology and can be compared rather closely. The 645D does use a CCD vs the CMOS sensor found in the D800e. That's a whole other can of worms that I won't get into right now.
Speaking from an iso range from 100-400 the Pentax gives very noticeable improved image quality than the D800e does. Not so much the megapixels, but the tonal and color quality is SO much better in my opinion; very noticeable in prints which is what matters to me the most. Most of the child photography was done with the Pentax 645D in my "kids" gallery. You can find reviews comparing the Pentax and the Nikon such as this one here so I'm not going to go into the technical mumbo jumbo that someone else can do better than I can. At this point I will never buy another new high end dslr for portrait work. The transition was about as great of the experience moving from my Dell to a Mac (lol, I had to throw that in there). As of this writing the D800e has been replaced by the Nikon D810 and the 645D has been replaced with the 645z.
The Pentax has a much simpler button layout which is HUGE for me when it comes to the speed of making camera adjustments. I hated the heavy menu layout of the Nikon. I like quick and simple when I'm shooting. I know you can customize a shortcut menu with the Nikon but why should a user have to go through all that? The D800e does have huge shadow detail that I love, but the 645D's sharpness, colors, and tonal range makes me forget about that quick. The D800e focuses faster and is much lighter vs the 645D's slower but more precise focusing and heavier body. There is no need for me to use micro adjustment with my 645D using my 45-85 and 55 lenses vs using the D800e with various Nikon lenses. The D800e is great for an all in one package but at this point in time, in my opinion, if you are a portrait shooter (who stays in the iso 100-400 range) the 645D is a better buy image quality wise. $3300 D800e vs $5000 645D is too close to ignore. My biggest 645D gripe is the image preview speed. It is SLOW but not Sigma SD1 slow. I use class 10 45mb-80mb write speed 32gb Sandisk cards, btw. A lot of comparison reviews state the image quality is too close to matter between the two. Let me just say this, my wife has an untrained eye and she picked the 645D prints every time vs the d800e prints of the same image. Every time! The prints have a 3d quality all the time that dslrs lack most of the time. That's the best way I can describe the difference in words.
If you ever want a 645D raw file to play with, shoot me an email. I don't have the D800e anymore to send you a comparison. Don't pixel peep, print!