You mean you were not using just the central AF point, in practical everyday work?
I've not found ONE camera where the multi-point system works well enough to be relied one. Sony's concept of 'zones' in the A77 is one of the best variants, with the three vertical zones.
I'm looking for focus accurate to the millimetre, not within feet or inches, and the D600 is delivering that just as well as the A77 or A580, or indeed the A900. Back focus problems with Alpha started with the KM7D and I was one of the first to try to method for adjusting the body myself, and writing it up; in the A100 and onwards, bad coincidence between the marked viewfinder AF point and the actual zone were added to the issue.
You've been very unlucky with Nikon. We have either owned or used for a reasonable periods many Nikons, every body since the original D100 in fact (but not the new 3200, 3100 etc series) including the 40, 60, 70, 80, 900, all 200s, all 1/2/3/4 series. Today I was processing some shots taken with the 300mm f/2.8 plus 2X TC on the D800 - a real challenge, action shots of a labrador splashing through the Tweed retrieving sticks.
My only error was to work at f/7.1 (just 2/3rds stop down from wide open with that combination) and ISO 200, the D800 would have been perfect at ISO 2000 in that light, but I didn't trust it.
Any softness is due to using a fast tele with a TC and not stopping down 2 stops - f/11 would have been better. This is the type of subject I routinely use for AF tests - this, or ducks taking off and landing on the same stretch of water. Plus indoor low light, outdoors at night, macro shots, landscapes with trees at 400 yards, etc.
I've had great results from many systems and individual cameras, and poor results as well. In the A57 Sony has produced a module actually much closer to a Nikon or Canon design than earlier arrays (all f/5.6, more cross sensors, no f/7.1 line sensors).
I actually need to calibrate our 16-80mm on the A77 as I've noticed that this is not doing exactly what's expected.