as far as Tom Cruise goes...while he does get "into" his roles 110% for his movies, I feel I am always watching "Tom Cruise" playing a character...I don't ever recall saying to myself, "wow, I completely forgot that was Tom Cruise in that role."
There could be 2 semi-exceptions that come to mind : "Tropic Thunder" and "Vanilla Sky", but even in those rolls, I still "see" "Tom Cruise" lurking just below the surface, despite all the makeup.
That being said, I feel the same goes for Clooney, Harrison Ford, and alot of the other 'big' hollywood stars.
I've always thought that if an actor is willing to "go the distance" and change their physical appearance for a role, they might have an edge as far as "acting" goes.
Look to Christian Bale in "The Machinist" for an example.