My roommate has an XRR, I've got a 610, and I've got a buddy with a 510. I like the 610 the best because it's electric start, it's a lot more compact than the XRR, and it's much more street-friendly than the 510. I've got probably 50% more fuel range than the 510, and quite a bit more on maint intervals too. The 510 IS pretty sick, though... I just like riding better than wrenching.
To me, the 510 would be great, if not perfect, as a second bike (I.E. whenever I want to ride around like a maniac and have a blast, I'll take the 510 -- when i want to go to work or out for a longer ride, I'll take the F800GS) --I just don't think I'd commit to it if it was my ONLY bike. I would feel like I could never ride it to work (~75 miles round trip) or just cruise around. Know what I mean?
My roommates XR is bulletproof, and has similar performance to my 610. It's uncomfortably tall for me (still in dirt trim), but other than that, I love it.
p.s. i don't own an F800GS, I just WISH i did. That's why I have a 610, not a 510 -- i put >200 miles on it yesterday.