Just got mine installed! Here's what I think:

I know why that little switch is golden. Because silence is golden. You can just about bunny hop your rig as high as you can and land as hard as you want, and no chain slap. On the trail, it is equally as prodigious.

I had it out on the Missing Link trail, as an out and back on the Tahoe Rim from Tahoe City. That trail is just so d@mn rocky! Here's what I heard: this one time, popping from rock to rock, I dropped especially hard, and I think I heard my chain slap... a little. I did pinch flat a Hans Dampf as a rear on a flat section. Fixed it right up with two of those miniature automotive-type plugs. Those things are awesome! Still, the deraileur was just unbelievably silent.

And yesterday, I had it out again. This time on the TRT riding south from Spooner, up to Genoa Peak by the King's Bench. Stunning ride, by the way. And this time, unbeknownst to me, I completely forgot to double-check my rig after putting on another rear tire, and left the darn thing in the off position. Guess what. Still pretty quiet. Quiet enough that I didn't really think about it much, only realizing this morning that I had left it off.

That tech should be on every deraileur SLX and up, soon, and for sure!

Thank You, Shimano!