When I was a despatch rider, I owned an old BMW R100/7. The handling and brakes were poor but I loved the 1,000cc Boxer engine. That was my 1st reason for buying the R1100GS - I love Boxers; just something about the noise they make, the torque they produce and maybe also the fact that no other bike manufacturer uses that horizontally-opposed twin cylinder layout.
Reason No 2 was the look of the bike. Love it or hate it, no other manufacturer has produced a bike so individually ugly-pretty since the first models were introduced in the mid-90's.
Reason No 3? It HANDLES. Despite the 19-inch front wheel and limited choice in tyres, you can sling the thing around like a super-moto. BMWs demand a smooth riding style, but once you adapt to Ze German Vay, you WILL give anything else on 2 wheels a run for its money on A and B roads.
The GS isn't a hard-core off-roader and it's no luxo-tourer either, but it was the 1st Adventure Sport bike, and it's still the class leader.
Front suspension, ABS brakes, engine, versatility
Poor finish, BMW dealer service costs