Having first done the ADV thing on a Yamaha XT500 with big tank, comfy seat and luggage all the way back in 1981, put 37 years on Harleys, and ridden 20,000 miles in the last three years on a GS... I am probably completely unqualified to answer your questions. Everything is too subjective! But here goes...
1) The GS is not a dirt bike. It is big, heavy, covered in breakable plastic, and a lot of work to pick up if it falls down on you.
2) That said, it can do incredible things off-road (for such a big bike). YouTube is full of videos where people do insane off-road stunts on the GS. Take a training class, even a short one (as I did last summer) and you will be amazed at how you can creep the bike along while standing on the pegs, making teeny tiny circles and climbing what seem like straight up dirt hills.
3) But it will be a lot of work. No getting around that; it's not a 300-pound dual sport. You will pay for your fun.
4) Even if you never take it off pavement, it's a fantastic standard or sport-touring street bike. Power, handling, braking and cornering clearance are all sportbike-caliber. If you have never ridden a bike with a Telelever, prepare to enjoy the total de-coupling of suspension, cornering and braking--hit the brakes while leaned over, and the bike just slows down. No front end dive, no sudden straightening-up and heading for the ditch. Wonderful. Plus, the off-road suspension soaks up the bumps (important if you live in a state that takes pride in its potholes), and BMW and the aftermarket offer all kinds of options for luggage and function/comfort/amusement.
5) ...if you can get comfortable on it. Depending on your proportions, this may be easy, or it may be a struggle. After three years, two seats, risers, and much fiddling, I have still not got the ergos on my '06 GS quite where I want them. I suppose it's the same with any bike, but I feel like I've struggled with finding the "comfort zone" on the GS more than I have on any other bike.
6) When you get everything sorted out (the bike, accessories, the gear you take along, your skills, your body), you can take a GS pretty much anywhere. With a GS, you can slam the long miles across the Great Prairie in reasonable comfort, exterminate chicken strips on the twisty mountain roads, and explore the unpaved-road world, all on the same bike. For me, living in the Chicago area and having an affinity for places like Moab, this is pretty cool.
8) It can be a mysterious and magical beast. All bikes seem to be going this way; BMW's just a few years ahead of the pack. While the Boxer is simple in its design, the GS, with its electronic suspension, CANbus electrics, riding modes, shift-assist, TFT display, and layers upon layers of software for safety and security, can be really incomprehensible at times. What did it just do? Is something wrong, or is it supposed to do that? Even though BMWs have outstanding reliability and longevity, it will take a mindset adjustment to ignore that little voice in the back of your head saying "I don't understand this bike; I don't think anybody does; I just know that someday it's going to leave me standing by the side of the road with a broken heart and an empty wallet..." Especially if you're coming from something as "too-dumb-to-break" as the Evo Harley and KLR650 that I had before I bought my GS!
Whizzy brakes, no FD drain plug, what's not to like?