2 cents of thinking
No I don't have one; currently riding a TT600R, so similar principle; single cyl thumper.
I agree about the F650GS - I have tried my wife's bike with her as pillion (as she won't take me pillion in any circumstances - who can blame her!). Just don't want to do that for too long or too far - crash helmets bump into each other because we are so close together.
So, for my 2 cents worth, I am giving lots of thought about why we have 2 singles in the garage!! The whole thinking is leading to the conclusion that this is not logical, and part of that is what you are asking. I'm inclined to think that a V twin around 650cc, or bigger of course, is the way to go - this points toward the Vstrom/Transalp world of course.
There are loads of factors of course and I am not sharing them all here, but you may want to develop this argument for your own circumstances.
ps I much prefer to ride the F650GS at speed (boring motorways and that sort of thing) than the TT600R - completely different characteristics of course. Also, the GS does around 70 mpg consistently (range = 170+ miles easily and loads left in the tank) with the TTR returning around 45 mpg or thereabouts (10 litre tank and somewhere around 100 miles to filling up again).
meant to say: it all depends where you are going and what you are going to do with the bike along the way and when you get there!!!!!!!!!!