She's a cultural icon, which makes her being in that role an intriguing juxtaposition, and she's an highly respected dramatic actress at the same time. I think some folks are letting the perceptions of Oprah giving away cars or letting Tom Cruise jump on her sofa obscure the reality that she could be awesome as Waller. Now Cumberbatch, he's OK, but nothing special, and is one of these overrated celebrities of the moment we frequently run across. Think Clive Owen about a decade ago. I doubt we'll remember that much about him 5 or 10 years down the road.
I don't care much about the Smith casting, but I'm sure he'll be fine, just like Cabbagepatch will be.