After purchasing this lens as an impulse purchase a few months back, I have actually been very pleased and impressed with this lens. Compared to older Nikon 28mm f2.8 AF-D which I also have, its bigger, focuses closer, is much sharper, has a smoother bokeh, has 1.3 stops faster aperture and costs 2.5 times the amount. Is it worth it? Absolutely!
I'll address all these issues more closely.
Size - In terms of volume, the 28mm f1.8 is close to double the size of the 28mm f2.8. But it is certainly not double the weight. The 28mm f1.8 is unusually light. In fact it is just over 50% heavier - 205g vs 330g. An extra 125g on a D600 is hardly noticeable.
Focusing - Actually the specs say it is the same, but as the front of the lens is closer to the subject on the longer 28mm f1.8, it certainly feels closer.
Sharpness - I haven't done a detailed comparative study, but subjectively this lens is amongst the sharpest I own, even in the corners, and well ahead of the 28mm f2.8
Bokeh - Sure, you'll say that the bokeh has to be better as the new 28mm f1.8 is almost a stop and a half faster than the f2.8; and while that's true, the f1.8 really impresses with its buttery smooth bokeh. Very impressive for a wide angle lens.
Maximum aperture - Where this really comes into its own is in very dark surroundings. Its amazing how dark you can shoot with this lens on the D600 and still get a perfectly usable image. A fast wide angle allows you to shoot almost a stop darker than a 50mm and not get shake. Handholding at 1/25th of a second is not an issue.
Downsides? None that I can think of, especially if you like shooting a 28mm lens. Perhaps size may be a concern to some buyers, but if you're shooting a heavy full frame D-SLR already, a 100g extra won't kill you.
Recommendation - If you find a deal on this lens, buy it!