I rode my 800GS in Spain a few weeks ago.
It is a very focused bike and will only truly reward those that use a firm hand with it.
Personally, i think the 650 in (much) more road oriented, while the 800 lives on sandy, pebbled mountain roads.
The seat is REALLY hard, the steering is a bit heavy but still "BMW" perfect.
Suspension and brakes are excellent and need no improvement (finally no need for ohlins!!!)
The twin engines are truly sweet. Loads of usable power & feels much more punchy than the claimed 85bhp.
Also, the exhaust note is a real surprise; deep tone with a touch of anger.
Here are some pics:
