07/19/12 DO NOT ATTEMPT THIS TRAIL WITH A BIKE FOR NOW. Done on 7/16/2012. Trail peters out after you leave the Switzer Falls trail, and disappears just after you arrive at the creek.
It *is* passable, but we averaged a half mile per hour lugging two bikes through, including a few waterfall passes, a jumping into a neck deep pool and tossing the bikes 15 feet into the river. Not something i'd like to have done alone.
Also the bypass around the Brown Canyon dam is almost impossible to find, and it has collapsed at one of the gullys near the top. We had to wedge the bikes between the rocks and use them as braces to continue around the dam.
Total time from Switzer to JPL was 7.5 hours.
Again, i'll repeat, this route is NOT currently suitable for mountain biking. We actually started @ Redbox, which is ridable to Switzer, so long as you don't mind the PoodleDogBush on the upper section.