Back in my Canon days I purchased an EOS 1D Mark III. Anyone that has followed Canon knows early models had a major fault in the AF system. Since moving to Nikon I've purchased 2 used D700's and new D300s. The D300s was purchased well into it's life cycle so I've had 0 trouble with it and the D700's perform as they should.
All that being said, I'd be hesitant to purchase anything brand new for 2 reasons. 1.) Like anything else, they take a major depreciation hit the day you put the first frame through it. 2.) As you've touched on, it seems like manufacturers are using the early adopters as somewhat of the last line of testing. I think it is impossible to think of and test every shooting scenario there is so until the camera gets out to the field no one will no for sure how it'll be have.
Me personally, if I decide I want a D600, I'll pick on up on the WTS forum in a few months and let someone else pay the depreciation.
"Sometimes I do get to places just when God's ready to have somebody click the shutter." -Ansel Adams