When I first moved to Toronto, I took my car here for some simple scheduled maintenance items (oil change, transmission flush etc.) and found the place really nice and service very friendly.
When I actually started experiencing some problems with my car (occasional stall, failure to start) I took my car here and it became a big problem. After three visits over three weeks with a whopping $1000 bill per visit, the problem was still there.
I eventually took it to a different mechanic where they were able to diagnose and repair the car in an hour and half for about $750 (parts included). I recognized this as complete incompetence on the part of Old Mill's mechanics and went to see the service manager about it. Nice enough guy, but unwilling to do anything for me.
Of course I can't recommend Old Mill GM as a place to get repairs done, but I had no problem with their basic services like oil changes etc.