Buy a BMW if you want a solid, dependable bike to ride for years.
Buy a Harley if you also want to buy a matching jacket, bandanna, t-shirt, beer cozy, key fob, boots, chaps, underwear, sticker for your car, bandanna for your dog, etc; and if you only want to ride your bike on nice sundays with a group of attorneys.
I still miss my '89 R100GS Paris-Dakar. *sigh*
From everything I've read about the current GS, it's an awesome do-anything STREET bike, but that it's too heavy for really serious off-road use. It will go further afield that any of the normal street bikes out there, but it's not made to be a dirt bike, really. That having been said, if the most "dirt riding" you'll see is the odd gravel or dirt road, it's supposed to be the best bike you can get (according to several magazines). A good alternative would be the R100R, which has many of the good features, and is usually about $4000 cheaper than the GS model. Sure, it won't go offroad as well, but it's also a lot kinder to your inseam.