Well the MoJavi works superbly. I carried 15 lbs of stuff (tools, tubes and spare fuel) and rode for two hours on an Enduro/hare scramble course full of whoops and I only had to snug up the left side once. After that it was rock solid.
Best of all I did not even notice it was there. I only felt the rear tool bag on my bum once while hopping a log.
This bag certainly voids the need for a rear rack.
The only thing holding me back were the god awful stock Karoo II tires. I have since spooned on some Maxxis Desert tires which are guaranteed to be an improvement.
The only caveat and this is mentioned in the mounting instructions is that the bag will scuff up your plastics and you need to install some sort of protective adhesive layer.