Received the replacement Sigma 150mm EX DG OS HSM APO last week. This lens seems to be very sharp at f2.8. I have my own routine for testing new lenses and this one passed easily. The lens is a fine replacement for Sigma's original 150mm f2.8.
With my 5D2 I can hand hold the lens to 1/25th second in flourescent lighting that requires F2.8 at ISO 3200 with -1 stop exposure adjustment. I do not know the lux value of that setting. I think that is EV -1. I do not think the OS is really good for 4 stops as advertised, but it is good for 2 stops for sure under these lighting conditions. That puts it on par with my Canon 300mm f4 L IS lens. I have no scientific evidence, but that's what it seems like to me.
The auto focus with my 5D2 is a little quirky under the very dim conditions described above. By that I mean that it I can not always use the center af point alone. I have not tried this same test with my 7D, which has a better autofocus system than the 5D2.