I'm replacing my FS100 with the A7S. Well, at least that's my intention. I just received the A7S and need to play with a bit more to be sure it will be a suitable replacement. And my FS100 needs to get sent to service to get cleaned up before I can sell it.
However, off the bat, I don't see anything that the FS100 can do better versus an A7S with a Juicelink for XLR and a small rig setup. Given that, plus the 4k upgradability of the new Alpha, I would be hard pressed to recommend the FS100 over the A7S except in very specific uses. Even in terms of perception, a rigged camera looks just as impressive as a "proper" video unit like the FS100 and of course there are times where you need to shoot on the DL and there the A7S has you covered. In terms of ergonomics, the FS100 is pretty terrible as well.
I can see why the FS100 is appealing at the new price (it is a fantastic camera), but with hundreds of hours under my belt with the FS100 I would have a hard time recommending it at this point, given the other options that are out there. (And, you know a FS100 replacement will be coming soon, now that they dropped the price - and chances are it will have the A7S sensor. Of course, it will also probably cost $5000.)