I picked mine up yesterday and was upgrading from my trusty old d80.
Quick bits of feedback as I've not had much chance to play with it yet:
Have to say the new body is a delight - good feel and chunky enough to feel like a real camera without being too heavy - that said, as with all my bodies a grip will be going on it ASAP. I just find they balance the body better and give me a bit more to grab on to.
Low light shooting is excellent as is IQ and noise control. ISO 3200 was clean and crisp when shooting indoors (birthday party last night!) and a quick test at Hi2 (25,600 or whatever it is) showed a useable (granted noisy) image that I would rather have than no image at all. For those that ever had one, the hi2 on the d600 is far and away better than the d80 could manage at 1600.
Jpeg in std looked so good that I am wondering if I'll now commit RAW to being a "just in case" backup for any images that need serious tweaking and that's saying a lot for me as I've been shooting RAW exclusively for over 5 years...
The manual is 325 pages long, so I expect it to be a while before I've figured all of it out