I'm not sure if there have been formal studies, but I know the sentencing circles are not used as much as they were in the past. Of course in some places it's not appropriate. It doesn't make sense in downtown Toronto. It only makes sense where there is a well-known community.
I know that Yukon or the Northwest Territories were using them for a while a number of years ago, but I think they're using them less and less now. I'm not sure why.