I have a '16 R1200GS water boxer, and still love the bike! I use mine mostly as a sport tourer that is great for taller/bigger people as it has plenty of stretch out room. Also, it has less fairing and windshield compared to the RT version of the boxer, and thus flows more cooling air. I like this as I live in the hot, humid southeast, air is good! I like the character of the flat twin, it has excellent torque down low yet will smoothly rev out and has good power on top. If I'm just loafing along, I keep the revs between 2500 and 3500, it will pull nicely out of a third gear corner with only 2500 revs on the clock. The 1250 shift cam version is even better, as the shift cam has both more torque on the bottom and better power at the top. My bike makes 125hp, the 1250 makes 136, on both bikes the power peak is around 7800rpm.
The chassis, telelever front suspension, and ESA adjustable suspension result in a very versatile motorcycle. Combine with the excellent BMW vario bags or the many fine aftermarket luggage options and you have a formidable sport tourer. When the going is easy, I put the suspension/engine management in ROAD/SOFT, set the cruise control, and enjoy soft ride that rivals any tour bike. And I should know, I also do own a tour bike (Goldwing), and the GS rides just as well. When the road gets twisty, a well ridden GS can keep up with most sportbikes, I put it in DYNAMIC/HARD, and the thing flies. A short vid of my friend and I, he's leading, I'm on the camera bike, both are GS water boxers:
The adjustable suspension also makes the bike very stable two up (with the right settings), and one can absolutely fly two-up if you have a willing pillion.
Me at Deals Gap on my GS, acting like a sportbike:
All loaded for tour on the Natchez Trace, bike on the left:
And right:
Offroad, with the right tires, skills, and attitude, it can carry you far off the beaten path. Yes, it's no lightweight enduro, but it is capable. This was on 90/10 tires, basically street tires, and it did fine.
Of the 21 motorcycles I've owned to date, this bike is by far the most competent and versatile bike I've owned, and I'm still absolutely 100% pleased with my purchase! If something happened to it I'd right back at the BMW store to buy another GS. Yes, it's an expensive bike, but good stuff usually costs more. Also, if you're handy with wrenches, the routine MX is quite easy to do, and a big bonus of the flat twin is the valve covers are out in the open making valve checks a breeze compared to in-line bikes.
PM me if you have quesitons.
EDIT: Lastly, I'd stick with the GS versus the GSA unless a 7 gallon fuel tank is a big plus for you. The Adventure version is a good 50# heavier, and carries that extra weight very high on the bike.