I just recently bought an '06 K12S after having put 30k miles (in 3 years) on my K12GT. One of the main reasons why I went ahead and made the switch was because I kept modifying my GT to be more "sporty". The second reason is because I am more at home with the seating posture and performance of a true sport bike...
Having said that, I test rode the "S" 3 times before I finally decided to do it. The first time I rode it, I thought that there should have been more "umph" to the bike than what I was experiencing during the test ride. The issue was that the power band in the "S" is about 2k higher than the GT. Once you get over the "buzzyness" of the engine, you will find that the engine revs up at about half the time as the brick.
Thoughts after 6 weeks:
1.) The performance of the bike, once you get used to it, feels like about twice what the GT is capable of. On the track, with my limited experience, I was eatin' up RS' and GTs like they were mopeds with a lot less work.
2.) The "S" feels like it is half the weight of the GT. As a matter of fact, just moving it around the garage seems like half the effort as the GT. I was always worried about tipping over the GT, but the "S" does not give me the same worries.
3.) There is always the temptation to throw you body around the bike when an on-ramp comes up with no cars in front... You just sieze the opportunities when you can. When there are few "opportunities", it gets frustrating.
4.) On the GT, a comfortable cruise was about 70 to 80 mph. Faster than that and you "noticed" the speed increase, even though it was comfortable. On the "S", you have to REALLY watch your speed. 65 mph feels the same as 85 mph if you are not paying attention to the spedo.
5.) The tranny... Here is where my GT will always have a place in my heart... I cannot find the sweet-spot shifts like I could on the GT. I have tried several methods (pre-shift pressure, clutchless, blurp throttle, decreasing apex throttle, etc.), but nothing makes for a smooth shift during normal riding conditions everytime. Occassionally, I can blaze through the gears smoothly up to 6th, but it's not the norm. It just wants to be pushed in order to get the nice, neat shifts, but there are few opportunities for this in everyday traffic.
6.) Have extra money set aside for accessories, tires, tickets, and radar detectors. You are now in the world of high performance and time-warp continums. Speed is relative and you need to be wary.
7.) The seating/handlebar position is no more/less comfortable than the GT. I miss the electronic cruise control, though, especially when I am trying to "behave"... I have gone 200 miles in a single sitting and I did not find it any more uncomfortable than my stock handlebar'd GT.
8.) There is almost NO hot air over your legs or boots on the "S". With a good mesh jacket and pants, you can easily handle up to about 98 degrees F very comfortably (in California). On the GT, I found that the threshold was around 90 degrees F, which is a big difference.
Hope that helps in your decisions. Just be aware that there will ALWAYS be some buyer's remorse and you will think twice about why you traded in your perfectly functioning GT for this twitchy thoroughbred "S". But treat the new "S" as the thoroughbred that it is and you will do fine.