Well, it's supposed to work on anything that can support OpenGL ES 3.1. According to Wikipedia this includes Fermi onward (although this fact is not cited).
I assume that on the AMD side, Vulkan will be supported by a great deal of cards that currently support Mantle (probably not all of them, as I bet some optional features will be promoted to mandatory).
I'm pretty happy with playing around with Mantle at this point, although getting shaders up and running is a big hassle since the AMD IL documentation is horrible and there are no public headers for it (to my knowledge). Plus, Mantle only supports a restricted subset of the full IL spec, and the only way to compile from HLSL is to use an old version of CodeXL since they dropped support for IL output in the newest version. You also have to preprocess the output file to extract the shader binary (which is given in comments off to the side in hex) since the textual output is completely wrong.