I had the Nikon 600mm f4 VR and 400mm f2.8 VR on loan from nikon for a month when i was considering a swap to them from canon, the image quality i was getting from them was on a par with the canon versions i have, I think the VR may have been slightly better than canons IS but that was expected as i believe its a newer variant, so i dont think you can go wrong with it quality wize, cannot comment about value for money etc as that's just a personal thing.
On a side note i didnt do a swap as for what i shoot i didnt see any benefit in swapping.