I also spent some time on a shiny new red K13GT today. David pretty much nails it here. The bike just flat works better and pulls harder than the K12.
The hesitation and flat spots are absolutely gone, and the bike pulls cleanly in any gear from pretty much any rpm. This bike had just finished its break-in oil change, so I was able to run through the full rev range with gusto.
Personally, I don't have any issue with the new controls, reservoirs, or side vents. All I really care about is what the next apex looks like as it hurtles towards me. And that it does, very quickly.
As you dive into the apex, the K13 leans in easily and stays rock solid until you rocket away with a growl. The changes to the trail and lighter weight suspension components do show when pushed hard and the bike simply responds without complaint. As it should do.
The brakes are the same excellent Brembo units fitted to the K12, and even full-on ABS stops from super-legal speeds weren't a problem. I didn't run that hard long enough to see if the brake fade problem is still there, but so far there wasn't any drama.
And I think they did something with the ergos (as others have mentioned) as I was pretty comfortable with the bars full up, even though I've been using bar risers on my GT for the past two years. Oh, and the stock windshield is only marginally effective for my 6' frame, but that's the same as before so no biggie there.
My two minor complaints (and they really are minor) are these:
There was excess play in the throttle cables that made 1st gear parking lot maneuvers jerky. That can be adjusted out though. And I don't like the new skinnier grips, but that's easily remedied with some slip-on foam covers (which also reduces any leftover traces of unwanted vibration).
Also, after a particularly hard full-ABS stop, I downshifted and let the clutch out while still rolling along a bit and I did get one loud, quick screech. No judder like before, but a bit of noise in an odd situation.
I was also able to review some secret BMW paperwork that described many of the changes in very great detail, with direct comparisons to the K12 and graphs, charts, and cut-away drawings regarding the new clutch, transmission gears, drive shaft, ESA II, suspension linkages, switches, etc. Basically, gearhead engineering porn. And it was very good.
I still have some concerns about overall reliability based on my past experiences, but I won't know for sure until I put 50-60K miles on one.
But even now I can see that BMW has gone over the entire bike and tweaked most everything. From the outside it looks very similar to a K12 but under the bodywork it's a whole different package that really shines as you're attacking your favorite corners.
We also had a C14 and an FJR on the ride, and met up with an ST13 later in the day. The bottom line is that none of those compare directly to the new GT in terms of fit, finish, factory farkles, and fun.
From all the reviews and specs that I'd read so far, it looked like the K13 is what the K12 should have been in the first place. Having ridden it myself, I can now see that that's really true.
So David, when do you pick up your new wheels, and can I borrow them for a while?