You can access Reiff Road from either direction. The Highway 16 end at the low water crossing still has a legal bypass available for motorcycles and quads. The gate on the west end (at the Lake County border) is fully open. Access down to Buck Island is open to any vehicle (though it is rough in areas).
Conditions are rough between Cache Creek and the Lake County border, but it is passable by any experienced rider on just about any bike (some slower than others). That will probably change by next spring, however. If you have further questions, let me know. Lets put it this way. I could still ride my Hawk GT the entire length, though it wouldn't be nearly as fun as my Adv. or 650R....
"I don't know what you do, but I know what I do, and I don't do that." --Uncle Doug, R.I.P.
"Without the possibility of death, adventure is not possible"--Reinhold Messner