Griffin,
As an ex fireblade owner and relatively new to this 'style' of motorcycling i can safely say that....
The R1200GS is a brilliant all-rounder. And idiot proof when it comes to riding fast. You have to jack the preload up on the rear for added ground clearance when really going for it on fast roads (not uk roads of course)
I have done 17k in one year on mine (normally did 7k/year on the blade).
That speaks volumes.
Of the 17k ive done about 10k 2 up abroad and the bike has never broken down but it has had its problems.
(final drive shaft/cam chain tensioner oil seal thingy to name the main two)
Due to the excellent but mind mangling expertise on this site, i have had phases of mechanical paranoia mixed with accelerated wisdom with how to get the most out of BMW and their dealrships.
The main thing to do is to have the dealership check that the bike has had all of the essential and recommended changes.
If you like the occasional tinker then the BMW is for you, cos there's always something to tighten up, or a little rubber thingy to press back on/replace etc etc.
If you like maintenance free, total handsoff motorcycling then consider carefully.
However, there's one big thing that overules all this, and that is the feel of the bike. And that's where the BMW comes into its own.
So, just go for a test ride. That's how i ended up owning one. I did it for a joke...
Who'd of thought it.
One final point. On the road, you'll always be quicker IMHO on a BMW than on a sports bike.