Even with my best handholding technique the image would still move in the viewfinder with VR on. The Sigma OS give a rock solid image to pull the trigger on.
Funny you say that. The final image may not be what is seen in the viewfinder. My 120-400 OS for example is supposed to have a 4 stop stabilisation system and it looks that way through the vf. in reality, I am lucky to get even 3 stops. Usually I get arnd 2 stops. The VR system I find is generally much more stable in practice than any third party system so far.