I owned a 2012 GS Adventure for about a year (19,000 miles of riding) between my FLHTK and FLHTCUSE7. Were it not for the fact that I had one that gulped oil and my bum knee, I'd still have it.
The GS/A is a great bike, and as you said, it put a smile on my face every time I rode it. It's really hard to argue with 9" of suspension travel, and no limit to the lean angle (you run out of tire before you drag anything). The Boxer twin is a beast of a motor. It sounds somewhat agricultural, but it pulls like a freight train and gets reasonable fuel mileage. The Paralever/Telelever suspension is just amazing....There's no front-end dive in corners, and you can trail brake the piss out of it. It'll flat out whip some serious machinery on a winding road. No slouch in a straight line either, but it's no 'Busa either.
And it's damn comfortable. The wind and weather protection is stellar. The suspension eats up rough roads. The riding position - once you get it tailored to you - is all day comfortable. My wife and I rode from MD to Utah and back, and would have happily done it again. All I needed to do was add a better seat (stock one's horrible), bar risers, and a pair of highway pegs. It's a different highway ride than a Batwing Harley, but I'd be hard pressed to tell you which one was "better". My GS/A didn't have cruise control. That was literally the only thing I missed. The kicker for me was my right knee. It got crushed in the wreck, so after a few hours it'd start to ache.
On top of all that, there's no bodywork to speak of, and it's built like a tank, so you can bash it around off-road without worrying about destroying it. It's too big and too heavy for serious off-road use, but for running down fire roads and two-tracks it's great (assuming you have the correct tires).
And maintenance is a breeze. Valve checks/adjustments take no time at all. The oil filter is pointed straight down as God intended. Transmission and final drive oil take no time at all. You can have the rear wheel off in less than 5 minutes, and you can do that with the bike on the center stand.
They're great bikes. When the 'CUSE7 is paid off I'll be looking for a used Waterhead Adventure to add to the mix.