I just had the opportunity to ride one yesterday. My buddy from university was visiting his parents for the weekend and invited me over for the afternoon. After a bit of talking about our bikes his dad offered to swap bikes and show me some local twisties to prove the competence of his gs. I was genuinely surprised how well it handled and the lean angles it could obtain. it was a bit unnerving at first, but once you get over the warbling of the tires, it just keeps leaning more and more.
Just like sbeau said, the suspension was amazing. I was aiming for bumps in the road, but the suspension soaks them up without unsettling the bike at all -even mid-corner. After ~200kms of windy roads, my arms were definitely more tired than they would have been on the 919 due to the extra weight, but the potential for long distance touring seemed almost limitless due to the great protection the fairing offered. While It was hard keeping up with the 919 on corner exit (and impossible when he uncorked the 919), the GS doesn't constantly tempt you to supra-legal speeds like my honda.
It was a really awesome ride with the linked brakes, the awesome telelever front suspension -which inspires you on corner entry, ABS -which I tested at 45mph on a gravel road, and other gadgetry but comes with a super high price tag. If I had the means, I think I'd switch up to a GS ADV like the one I rode. I mean he's owned his for 4 years and put on 70k kms with absolutely no mechanical failure whatsoever. The frame and suspension still all look great even though the guy never washes his bike. So I guess you're paying for build quality -not that you can't get that with Honda for much less $$$.
By the end of the ride we had traveled nearly 400kms, and aside from my slightly tired arms from the corners, I felt fresh and realy to keep riding right through the night. I would suggest to those considering one, that a quick test at a dealer will leave a sour taste in your mouth if you've just come off the 919, you really need to take one out for an entire day and try it on the freeway, country roads, and even some gravel to appreciate the versatility and quirky nature of the bike.. Again, like sbeau stated, the power delivery is completely different, and it DOES sound just like the old john deere I used to drive to mow the ditches for the county -albeit a little faster.
But the low maintenance and versatility left a bug that I just can't shake.