I use my monopod, primarily when I use the Bigma in dynamic environments, where light is not an issue but I don't feel like lugging that thing on my arm all the time. And I just got the Sigma 70-200 2.8, which is also not the lightest piece of equipment. I have also monopoded that, and it's worked ok for me.
I would have to say, that if handheld provides (and I am just establishing a relationship, not study numbers) about 50% of Stability, and the Tripod somewhere between 90-100%, I have found the monopod to be around 60-70% stable. Personally it keeps me level, but the movement -though reduced- simple varies from side to side, which does not matter in panning situations.
If you do not need to be moving (say you set up at the outside of Monza turn 3), then a tripod would probably be best. I defined the need for a monopod once I started tripping with my Tripod, as it was a hassle to fold and unfold every five mins, and it was no fun moving it around either, specially in crowded spaces.
But by no means is the monopod -IMO- the magic staff of stability.
My Thoughts.