I have had the newer OS version for a couple of years and have found it to be a very good lens. I shoot Nikon, so it's been paired with a D300, D700, D7000 and most recently with my primary body, the D800.
It's not a fast lens, nor, from a Nikon user standpoint, optically on par with their new 80-400 but with greater range and $1,000 less, one really has to want the Nikkor. As I already have the Sigma, I've so far passed on the 80-400, at least until the price comes down. If discounted enough, I imagine NAS would kick in!
I suspect that better optics is true with Canon's 100-400 as well but I cannot say that from personal experience and again, the Sigma has more range and a better price point.
Given decent light though, I've found the auto focus to be very responsive and the OS is worth 3 stops (+/- 1/2 stop).
At approx. 4 pounds, it's not light or small but its versatility and generally good optics are unmatched by any other lens that I'm aware of, especially at that price point.
I hope this helps,