Hi,
I don't know if you've made the jump yet, but here's my opinion, fwiw.
I changed from an '07 RT to the K1600GTL recently, and I have to say I'm vey happy so far. It's an incredible machine and very capable, far more than me, but I hope to learn over time.
Strangely, the change was for the partly for the benefit of my SO (honest). She found the RT to be a little difficult to mount & dismount and the seat to be uncomfortable after a short time (she has 2 fused discs). On the test drive, she found the K to be much easier mounting/dismounting and far more comfortable on a run. She found the seat & suspension (admittedly on the 'comfort' setting) to be a big improvement over the RT. Incidentally, we took a long test ride on a Goldwing and although the seat was probably the best of them all, the suspension was intolerably 'crashy' on our UK roads, and gave her too much shock through her back.
For myself, I find the K1600 to be superior in all departments. It's much more comfortable for me (6' 1'') and the pegs don't bend my knees up as much as the RT. The engine is addictive, with a howl I didn't expect in the upper range. It'll pull happily from low revs in all gears and it's the smoothest of any I've ever owned.
It's stable at speed and is as nimble as the RT, if not more so.. I'm getting used to the electronics & displays and although not entirely "intuitive', it's still pretty good and fairly easy to find my way around.
Downsides? Not many so far. The luggage capacity is a bit less than the RT, it would seem. The top case isn't as 'planted' as the RT, and it's very slightly down on MPG (46 as opposed to 50 on the RT), but it's a bigger engine, so I still think that's still pretty good. The SO found a bit more buffeting at higher speeds, but at and around legal speeds, it's fine. I felt that the weather protection wasn't as good as the RT, but I rode through a British deluge the other day and apart from getting wet shoulders, I stayed dry, much to my surprise. There is some definite 'shunt' in the drive that others more expert than I have commented on, but I'm guessing that's a combination of new engine, transmission and my ineptitude. However, I'm beginning to learn how to reduce it and remove it altogether with careful driving & shifting.
So happy with my purchase? So far, very much so. I originally came from a LT to the RT and always felt that although it was a fine machine with lots of character, it was just a bit too 'small' for me. The GTL now gives me back the feeling I had with the LT, but it's a much superior machine to both the LT and RT, in my opinion.
Apologies for this being such an anecdotal account, but I haven't had it too long (less than a week), so I haven't been far enough on it. But as a final indication of my current impression, I felt confident enough to take it up to and into central London to visit an Olympic event on the 2nd day of ownership. It was a joy to ride and I felt perfectly comfortable in amongst the traffic and crowds. If I'd had any doubts, I wouldn't have gone near the city, but it was fine.
So, take a test ride, but a word of caution. If you're in any doubt beforehand, leave your wallet at home, because there's a high likelihood you're going to like it and you'll want to buy it on the spot.
I could post a more technical appraisal as the miles grow.