I have had a 13 &15 GS for past 2 years. Rode the 1290 last week, just for an hour.
I found it pretty comfortable, easy to balance and handle and quick to turn, uber powerful when needed, surefooted and well planted.
The seat height was a bit higher and I could not quite flatfoot it which doesn't help. Of course it is shit quick when you need it and fairly easy to handle when giving it some and the front does not dive much under heavy braking. The modes and controls/menus are fairly easy to access and change settings, even on the move, not as easy as the GS with dedicated buttons though. The seat felt a bit like sitting on a flat surface and I imagined it would become uncomfortable after a few hours?
The engine on the 1290 was pretty smooth considering it's V twin config. I did not experience any obvious wide range vibes in any particular RPM range which was good. Gear changes were smooth, brakes are Brembo so no problem there and the electronic suspension did a great job of keeping the bike level fore and aft under loads.
When I got back on my 15 I did not miss the power of the KTM and the GS still just has that all round comfort with enough power and sharp handling. The 1290 is a great bike, I reckon if I rode a 1290 that I could wean myself off the GS in about a week and my view would be completely different
Taking the S1000XR out next week, my XR allocation is due on 12 July, but I might stay with the GS, see how she test rides I guess.