Render finished, so I did the "strong backlight" test. Backlit only, spotlight w/ depthmapped shadows. Here you can definitely see a difference, at least with my monitor's settings. (If both look black, try temporarily increasing your monitor's brightness to see what I'm talking about.) The SSS has the face in shadow properly, while the ambient makes in visible in an oddly alien sort of way. It's a shame there isn't more control over the Fastscatter effect, however. I wonder what happens if you throw in some tranlucence? Hmm...