The more I think about Mantle, the more I dislike it. I know the politically correct thing is to say you hope it does well, but it seems like a slippery slope going in the wrong direction.
You start with AMD making an API only for their GCN hardware to give close to the metal coding. Now Nvidia has to do the same to keep up with AMD. Now Dev's have to code for Mantle, DX and Nvidia's current arch. Now Intel steps in, and we have Mantle, DX, Nvidia and Intel API's. Then sometime down the road, AMD changes direction on their architecture, and they have to make a new API, and now we have Mantle, Mantle 2.0, DX, Nvidia, and Intel's API.
It would be fine if we replaced DX, but going close to the metal for specific hardware is not a good precedent in the PC market. It's the start of things going in the wrong direction.