Way too much street bias for me. If I'm going to ride "just" a 650 adventure-tourer/dual-sport machine, I prefer something that is more dirt-oriented because it can be light enough to actually be fun and fairly capable in the dirt.
In testing the F650 for a couple weeks, I found it to be a decent street bike with good attempts to integrate several BMW features, but when I took it into the dirt, it wasn't so pleasant. The tank's shape, especially, comes to mind. It's sculpted with nice scoops that are kind of handy to tuck your knees into when sitting down (as you would be on the street), but when you stand up (as you would for any sort of aggressive dirt riding), those scoops become very painful as you try to grip the tank with your knees--and I wear knee braces!
The suspension was nothing to write home about either.
Personally, I'd go for something along the lines of KTM's 640 LC4 Adventure if I wanted that engine size and was thinking of something that would also be street-legal. It's got a huge tank for great range, but you can still go pretty quick in the dirt on moderate single-track. For the more gnarly stuff, I'd chose the 625 SXC and hang a Baja Designs kit or something like that on.
But again, I'm more of a dirt guy first. The F650 is pretty smooth running on the street. [:)]