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I second all the good comments about the Contax 645, but agree that the RZ67 and 6008 systems are also worth considering (although the Contax is much more compact than those two). Note that the Contax has a regular shutter, not a leaf shutter, so if you want to use flash, one of the other systems is better. If you don't, that weighs in the Contax's favour.
The best lenses are the 35, 55, 120 Makro and 350, which are all brilliant. The rest are merely good I shot one project exclusively on the Contax 645 with 35 and 120 on Adox CHS 25, and got great results. The lenses are really great. The 35/3.5 is the medium format version of the 21 Distagon, btw. Same design, same great results, but with less moustache distortion, I believe.
Get the waist-level finder, if you can find it. It is great for tripod compositions. Buy lenses with the hoods, if you can find them, since the hoods on their own can be quite expensive.
Sadly I haven't used my Contax system for some time now. I also own a Hasselblad 203FE as well as digital, and somehow I don't pick up the Contax any longer. I should really pick another project to do with it, as it is such a pleasure to use, simple and perfect.
I'll second all of this and add the following: The 140mm and 45mm are better than merely "good". The 140mm is a lovely portrait lens. The AF is a bit slow by today's standards but was better than its contemporaries when it was made. If you are familiar with Contax 35mm SLR cameras, then the 645 will be very familiar to you -- its handling and control layout is almost identical. It is a very ergonomic camera and intuitive in use. Finally, it's shutter and mirror are much better damped than its competitors --certainly those competitors when it was made (I spent a lot of time with both the Contax and the Mamiya at the time and the Contax was superior in virtually every way). My understanding is -- although I don't have first hand experience -- that it's still better damped than current 645/6x6 format cameras. I loved using the 645. Sadly, I too don't really use it anymore -- and I have a freezer full of 120 and 220 film. Maybe I should dust it off and take it out for a spin.