The rear shock mounting point on the "s" can crack due to the damping being a bit too heavy.
When the shock heats up it loses it's damping capability so the back end becomes a pogo.
Some people change the rear rotery damper for a more conventional type but that'll cost you around £4-500 if you don't ride hard it may never be a problem but pillions do cause the pogo effect after a while on bumpy roads.
I also hear negative things about the "r" but it was designed to race and therefore it's a bit twitchy on the road, i'm selling mine cos "it scares the **** out of me" and the 916 feels like a pussycat.
