i wouldn't do it. that back was never fantastic, even when it was one of the only ones of its kind. the sensor is the same size as a 35mm piece of film if you include the perforation holes (cut in a square). quite a bit bigger than a 50d's sensor and bigger than a FF dslr, but not that much bigger.
i am making an assumption that the body you would get is the Mamiya 645afd with a replaced focus screen with square crop marks. when i shoot MF, 6x6 is my favourite format, however shooting a square crop in a 6x4.5 viewfinder will be really odd and you will make quite a few framing mistakes. this back also does a weird 4x5 crop, which is pointless to use. the body does not auto focus very well. i can't remember on the original 645afd if it has 1 or 3 auto-focus points. i currently have the afdiii which only has 3 points, and i always focus manually. i'm sure it will come with the 80mm 2.8, which isn't a bad lens. i'm taking a punt that you'll also get the 55mm 2.8 and the 150mm 3.5, which are also nice lenses.
if you are getting it with the contax body, i can't comment as i never shot it on that body.
the dcs 645 back is *really* slow. take a picture, wait for it to write. a couple of seconds later, take another photo. if you want to see the image on the screen, then wait for it to be processed. while this is fine if you're using it in a studio or doing portraits, compared to the 50d, you will find it frustrating.
the good thing the back has going for it is it's daylight IQ. shadow detail and colour are fantastic - especially for portraits. compared to other digital backs, the high-iso noise is pretty minimal, provided you slightly over-expose your images on iso 200 and above (it only goes to iso 400...).
again i wouldn't trade. if anything i'd be saving my coins to get a full frame modern dslr instead of something that uses 8 year old sensor and processing technology. any difference in IQ can easily be made up for by investing time in your shot compose properly, expose to the right, consider your horizon, check your backgrounds, and bracket if necessary.
