I had a brief ride on an S10 a few days ago.
Impressions:
Comfy
Grunty
Smooth but still with power beats in the background
Amazingly low c of g
Roomy
Easy to ride
Not much to bag out

I've ridden both the R1200GS and GSA. I think the S10 is as good, if not better ergonomically. It also seems as grunty. Unknowns (for me) are durability, real-world fuel efficiency and ease of servicing (a big plus for the 660).

The 1200 is probably worth the money if it can take the punishment of everyday living and tough touring.

My opinions for what they're worth.