How many graphics card drivers are released with claims of "up to 45% faster"? How often do people in real world situations get an even noticeable improvement? As a result (assuming the answer is "essentially no-one"), how excited do you get when the next drivers make similar claims?
Also, how often do you hear about new techs reducing the overhead for cross-platform development? I presume the PS4 doesn't use DirectX but then AFAIK Mantle only has 3D stuff; 3D is where performance matters most but there's plenty more that needs handled.
I'm not going to hold my breath that Mantle changes the world on either count.
Competition is good and for whatever reasons, OpenGL isn't really providing that. The fragmentation of development isn't good but that's a necessary evil for the time being.