I've ridden the R1200GS's extensively, and have been riding dirtbikes for nearly 40 years now. Streetbikes for about 30 years. As previously said, the R1200GS is no motocross bike. Graded and/or washboard roads, and not too rough or tight two-track are it's element. Sand dunes can be fun to blast along on, but anything technical like big jumps, or carving bowls, is best left to real motocross bikes.
A technical two-track quickly becomes a real pain on a top-heavy and tall bike, that when loaded is pushing the 600 lb range.
The new partially water-cooled R1200GS definitely has more power than the old oil/air cooled GS's. It now also has a wet clutch in a shared sump design. Based on that alone, I'd wait a bit to see how their long-term reliability is. Especially since newer BMW's of the last 10-15 years or so can be somewhat finicky reliability wise, anyway.