I have had the Sigma 50-150mm F2.8 OS version about four years, after having the non-OS version II, and reading a ton of reviews on the OS version prior to purchase. The only reason I bought the OS version was for the OS. The non-OS version II was/is great. Try as you might, I doubt you will find anything but excellent reviews/comments about the OS version, which is what I found. Regarding comment about the non-OS version II, I also found it extremely sharp and quick-focusing, as well as much lighter than the OS version. I use/used each on Nikon DX cameras as my most frequently used lenses.
The OS version is sharp as a tack, as they say, focuses very quickly and accurately, and also works great with Sigma's 1.4 TC. I have shot sports, BIF's, events with quickly moving animals, etc. and the lens just performs perfectly, as long as I do my part. The 50-150mm F2.8 OS lens uses the same size barrel assembly as Sigma's 70-200 F2.8 OS lens, and it is significantly larger than the non-OS lens.
I have yet to hear of any actual owner of this lens have anything but very positive comments on its performance. I did have to send mine in to Sigma for re-calibration to AF properly on a new camera body I bought last year. I called Sigma, shipped it to them, and in a three day turnaround, had it back fully functional and at no cost other than my shipping to them. I have several Sigma lenses and to me their customer service is superb.
If you can find one, I think you would be very satisfied, provided the focal length works for you. Good luck with it.
