Over the course of a couple of weeks we put 3k miles on a K1600GT in California and Oregon which gave us a pretty good road test.
We did extremely twisty coastal and mountainous roads, we did flat valley 'get there' speeds and a lot of non-pavement roads. Overall, just on riding performance we would give the K1600 a grade of C. We have put 115,000 miles on FJRs and would give them a grade of B+.
The saddlebag space was essentially the same as the FJR, we used our FJR bag liners in the BMW bags and they were a perfect fit. The passenger seat didn't feel like it had the same room as the FJR. The stock seats sucked. Really sucked. I didn't prefer the spacing of the lower transmission gears but huge torque made up for a lot of that. After a few hours of whipping that tank around extreme corners with a lot of vertical grade change the weight and size of the bike became a liability, it started to become tiring work in the hot weather. I did lot of shifting of the 6 speed transmission. The K1300 we rode earlier had the clutch 100% engaged when the lever was 0.100" off the handlebar, the K1600 didn't have the clutch engage until the last 0.100" of lever travel, though this may be unique to these bikes and some maintenance would fix the issue. I ground up the right peg feeler. I ground up the left peg feeler. We went for a ride through the mountains with Bugnatr and ground both peg feelers and the center stand too. Similar speeds through the corners on the FJR would not have had as much metal removal but to be fair, my FJR had an aftermarket suspension. Gas mileage wasn't inspired but the roads, gradient changes and my sport oriented riding didn't help.
Pillion gave the electronic suspension two thumbs up, so much that we now own a '15 FJR ES. The TPMS was nice to have, especially since we made almost the entire trip with a big nail in the rear tire. The engine made huge torque but didn't seem to hustle the bike much quicker than my '04 FJR. I got used to the pages of selectable dash menus pretty quickly. I learned to like the selectable modes like Tour and Sport. Then there were those silly little plastic pop-out air flow wings to divert air. Once the temps got over 100° I figured what the heck and popped 'em out. Boy howdy! The wind whistled up my crotch, under my coat inflating it like a balloon and shot the air into the chin opening of my helmet causing it to try to lift the helmet off. If I could find a way to make them work on my FJR they would have already been there. There isn't much else positive that stood out or was significantly better than our FJR for half the bucks. BTW the K1600 only had about 40k miles and we were comparing it to a 100k mile FJR.
I tried to love the K1600 but never managed much more than a neutral view. Pillion just said NO to both the K1300 and the K1600. If someone were to offer to just give me either a FJR or a K1600, in the end I'd probably would choose the FJR and without hesitation I'd choose a Gen III and higher FJR over the Beemer.