Hi there,
I've been using a 35 adapter with nikon SLR glass. It really depends on the adapter but they all cut some light. The best I've seen (guerilla35) only seem to cut about one stop. Mine, on the other hand, cuts about three stops and is a lighting nightmare for inside shots.
As far a follow focus' are concerned. There isn't a lot of options yet, but they are coming. The folks over at indifocus.com have promised a version of their FF that will work with an adapter and nikon lenses. and the Cimtech crew also have one in the works.
And as a general note on these adapters...they are essential. I can't shoot without them anymore.
-rook