I did some miles on the previous incarnation, the F800ST. A cracking bike. I believe the engine is tuned differently but mechanically much the same as the 800GS.
Agile, good handling, comliant suspension good at small bumps but a bit harsh on bigger ones although I think that is one thing BM improved on the GT. Fuel consumption brilliant. Fairing was a bit spartan and the screen too small for distance at speed but these are also changed/improved on the GT
+1 on crap mirrors. The standard, half fabric, expanding panniers are also crap.