I have a F650gs twin with ABS, and I usually turn it off whenever offroad.. However I noticed last time I was out, i kept stopping and starting and left it on at one stage, down rocky and muddy terrain it really comes into play with the standard gearing, you see std gears arent low enough to provide enough engine braking going down hills but keep ur foot on the rear brake and it works awesome! keeps the bike at total idle!
But id still recommend it to be off on gravel roads as it just doesn't pull up on them.. a simple stop n hold ABS button turns it off so not a big issue, i wish you could do it while riding though,
So ABS for mud/hard rock = great!
gravel = not so great!
my 2 cents..