Just had our car out and the stability control didn't interfer to much. I took some corners harder then normal in the snow and got on it alittle. The rear would start to come around and then it would straighten itself out. Car handled really well.
So I would say it kicked in when it should. However if you really want to have fun sliding the car around I could see where it would be annoying.
On a side note, I felt the stability control in my Focus worked better then the Impreza. At least on the corner I took the Impreza seemed to understeer longer before the system kicked in. However they seem to allow for about the same amount of rotation before kicking in.