I own the 70-200mm F2.8 EX DG APO Macro HSM II (I paid $1600 for a new set. T_T) and like you, also use a 5DM2. It's a nice combi.
The sigma is pretty decent. I got a good copy so it's pretty sharp. Focusing is quiet but will have some problems when it's dark. Has a slight orangy tint to it.
As for IS, it is not a must but situations do change and when i dont have a pod, that's when i REALLY wish i had IS.
If you want budget, the new sigma 70-200 has OS on it.
Bottom line, you have to ask youself these questions:
How much am I willing to fork out?
Am i willing to sacrifice quality over price? and vice versa.
As for me, I'd save up and get the Canon with IS.
My reason; I get quality glass and peace of mind.
thanks for you inputs. really helpful.