Blender 2.78 got some pretty nice features which make it a lot easier to make IBL with nodes.
And IMHO the Result I got is quite decent (here is the .blend: https://www.file-upload.net/download-12244099/RealtimePBR.zip.html (the original HDRI is from there: https://www.hdrihaven.com/hdri.php?hdri=white_cliff_top) ):
However how I achieved the Specular highlight is dirty and (probably) not very fast, I just mixed several materials:
but it would be much nicer if it would be possible to control the hardness with the extended material node because it’s faster and you’d have a smoother transition at varying roughness levels.
The second ‘issue’ is the interpolation at cubemap seams (very extreme example with a 6x4px cubemap):
Because there is none xD (In Blender Render there is but it is interpolating it ‘wrong’).
If these two issues could be fixed PBR in the Blender Game Engine would be -almost- perfect, so if you know a way how to fix one of these, please let me know ^.^