I tested the 120-400 OS here
. The 150-500 OS is the same construction just with a different front-end. I was quite disappointed by the efficiency of the OS but different opinions on the web suggest that this might have been a QC-issue (not the first with a Sigma lens).
If the OS works properly the stabilization could be very helpful when shooting the long end w/o tripod.
But if you normally carry a tripod with you you might like the non-stabilized older 170-500 version.
Thomas (beware: Nikon-fanboy and moderator!) My Pictures
, My Photography Blog