Interesting range - do you have a personal preference between 150 and 200 ? The more I think about it the less I like the focal length but maybe it is pretty close in actuality ?
Right, it more or less behaves like a 70-200mm lens on FX. But then one could of course also mount that 70-200 lens on a DX camera.
I must admit, that my first thought when I unpacked the 50-150 OS was "Fail!", just because of its size and weight. But during the time I had the lens I shot (among other things) a concert, where the 50mm short end was very convenient. A 70-200mm lens on DX would have been too long at the short end. I assume I would appreciate that zoom range on DX during a wedding, too.
So, it very much depends on the use case one has (as usual). Just be aware that there is no size or weight advantage with this lens. Especially if you take the DOF-potential into account, one would have to compare the Sigma 50-150 OS with the Nikon 70-200/4 VR. The latter on a D600 is a less bulky combo than the Sigma on a D7000 or D7100.
Definitely interested in this review. Like others, I initially reaction was to the unique focal range and felt it was a bad range. Then I thought about it and realized if it behaves close to a 70-200mm on full frame for a crop camera then that's perfect, my 70-200 is definitely too long on the wide end at times. The ever popular 17-50s for crops work close to the 24-70s on full frame and those are picked up en masse.
Heck if it delivers IQ similar to the flagship stabilized 70-200 f/2.8s, then at half the price we might have a winner.
P.S. I shoot Canon, but I know the review should be more or less similar.