From someone who always rev matched to down shift on the road, I love the slipper on my Gsxr track bike (yep it's stock and works quite well). They aren't necessary but they come in hand once your pace increases. They aren't "marketing hype". The 600rr (Honda) is the last 600 to give you a slipper from the factory (is it standard now?)and when your prepping a bike for the track (where 600 ss are suppost to be) its an additional $700-$1000 added to the bill (if your slow enough to need/want it

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I went from an SV650S to the first production slipper clutch on the 2005 ZX6R, and it was no gimmick! Damn near a revelation. Though I can't say I tracked either bike, but I used to bob and bounce that SV back wheel into all my stops (mostly for entertainment - it was so damn fun). Rev matching wasn't a problem, but it was done 1 gear at a time (in quick succession). But I also loved extreme hard braking and having the back squirmy.
With the new (at the time) slipper clutch it would literally take a massive drop in say 5000rpm (possible 3 gear differential) to unsettle the rear for even a brief moment.
By the time I was tracking my bikes I was on a ZX10R with a slipper as standard, and though I rev match, there were times in a panic when I just mash down the gears, get a little wiggle and bam, right on track for the corner. It comes in handy.
It is a trickle down technology from the big boy racers, and it has it's advantages.
Deep down I miss anchor toss that was the V-Twin engine braking / non slipper clutch that was the SV650S and pine for it. Not so much to say an 1198 would be effective on the track, but enough to really enjoy the Panigale for instance.
Engine Braking Controls can smooth out entry in addition to the slipper clutch (in race mode), or with that monster back torque you can still get the rear squirmy with a single gear downshift and no EBC, with a mild rev differential.
I would say that is no gimmick. That being said, the RC8R has none of that and gets decent lap times....but then again, that engine is mechanically tuned to be quite mild and smooth.
On the race track, the slipper probably has its fans and haters, but on the road the slipper is truly a safety feature that's worth having.
Ok back to turning techniques.....