Depends on the game. Some (most) games don't even have DX11 support.
Even with the ones that do, only a few will you see a big differences in graphics quality, Metro 2033 for example, and of course DX11 benchmarks.
DX11 is still immature, however the possiblies of DX11 are amazing, and in the future there will be a very big difference between DX11 and DX10.