I just purchased a GT after having possession of a demo for about 24 hrs. I'm 5'10", 200 lbs with a 31" inseam and commute year round here in Seattle. The bike I rode had the 'comfort' seat which is supposed to be about an inch taller than the standard seat.
First of all I found the ergos perfect for me. A slight forward lean with my feet under my hips - very comfortable for my back. The fairing provides excellent coverage of my body while still allowing my head to be in clear air. It was foggy and drizzling on my morning commute, and while I stayed dry, the wind was enough to blow my face shield clear. Interestingly, the rain rolling up the windshield was somehow channeled to the sides rather than into my face. I foresee no need for changes to seat, pegs, bars or shield.
The bike was very light compared to the RT - there is probably 100 lbs or more difference. With the fuel tank under the seat the weight carries low. It's also a very narrow chassis. The exhaust note sounds funny to me - reminiscent of my DRZ400 thumper - kinda farty. Actually even though I mention it, sound is about at the bottom of my list of priorities. There is plenty of power for maneuvering, merging, passing - but I'm no racer so I don't know if it will readily pull wheelies (with the traction control off of course). The brakes are very powerful, and if you start braking with the rear and then apply the front the chassis settles down nicely. I ran it up to 80 mph on the freeway and didn't feel any vibration to speak of. The view in the mirrors at that speed was clear. I expect to get between 50 and 60 mpg with it.
Speaking of mirrors, they are not so wide that you can see anything immediately behind you but they are certainly no worse than the RT rear view.
I didn't have the bags although I've seen them and ordered them for my bike, which will be the Light White. They are definitely a little smaller than the RT system cases. I didn't test the ABS, or the alleged problem where the computer will release the front brakes if it detects what it thinks is the rear wheel lifting. I didn't test the traction control either.
The ESA only changes the rear shock damping and you can definitely tell the difference between the two extremes - sport and comfort. Everything else - fit and finish, controls, instruments, bike computer, heated grips, etc are typical BMW. There is some heat coming off the frame spars - I was in a 40 min traffic jam creeping along in 80 degree F temps and it was not a problem for me in Dockers. If I was wearing my aerostich I probably wouldn't have noticed it at all.
The belt is quiet with no off-on transition snatch. It's supposed to cost $400 and be replaced at 24K miles but reports are that most people far exceed the 24K mile life expectancy. It is also very simple to replace (especially as opposed to a chain). Other services (oil change, battery and air filter access, etc) are very straight forward. Supposedly, the diagnostic computer will tell the service tech whether or not the valves need adjustment (don't ask me how THAT works). The rear wheel can be removed on the center stand without removing the muffler.
It's not cheap at just under $13,700 with side bags, the safety and comfort/luxury option packages, freight, and prep. My '09 RT was at least $5K more than that. A lot of my leftover RT stuff like the GPS power connector, battery charger, oil filter wrench and axle wrench will work on the F800GT. I wouldn't necessarily recommend this bike to someone who does a lot of 2-up or super long distance touring, or is much over 6' tall.
I also looked at the Honda NC700 series and the Moto Guzzi Norge. Neither one is in the same league as the F800. It's really in a class of it's own at least right now and if the height and weight and fuel economy of your RT is getting you down it is definitely worth your while to arrange a demo and try it for yourself. I'm calling it my Wee-T!