My experience is pretty much like Jimmy's. I have .5 diamond on one side of a SRD hanging felt strop and .25 on the other side. The felt seems flat to me, at least as flat as an unsupported piece can be. It works great on my razors.

I also have a flatbed hone with hard felt pads treated with diamond spray and CrOx. The pads are not flat in the sense that a lapped stone would be, but because of the slightly springy nature of the hard felt I don't think it matters. That is, I think that the resiliency of the material makes up for slight imperfections or curvature in the stropping surface.

Good stuff, this hard felt.