To add to RTW Motorcycling's excellent feedback, I've just finished my first significant trip (Bilbao to Perpignan through the Pyrennes - ride report to follow) on the F800GS and can confirm my own comments on some of these issues.
The seat is firm but not uncomfortable (for me) but as planned, I got the lower seat and have fitted an Airhawk (cruiser, standard size) for this trip that fits pretty well into the recess. No problems in the derriere!
I've had one 'stall' in 1200 miles, changing up from 4th to 5th, accelerating at high revs but I realised as I let the clutch out so was able to restart the engine while coasting. I'm a little concerned about a reoccurrence.
Unlike RTW’, on the gravel roads above Sort in northern Spain I found the bike a real handful; possibly partly my own inexperience but I'm certain the standard-fit Bridgestone Battle Wing tyres contributed significantly to the lack of handling. Kept it upright but made me sweat!