I think this 12-35/2.8 and the other 35-100/2.8 are two must buy lens, at least until perhaps Oly makes a better 12-35/2.0 at reasonable price.
The larger and constant aperture is very handy in in-door and evening lighting situations. Although the it's not comparable to APS-C and FF DOF, in reality you don't really always need such shallow DOF. When you want that, one can always use 25/1.4 or 45/1.8, and it comes in a lighter and smaller package overall.
Well, this may not be an attractive camera to mirrorless camera owners, but it would be a good backup camera for existing Canon users.