Not that this is relevant, but I got to ride a 2006 Husky TC510 today. Very sweet bike!
I went right from my KTM 300EXC to the 510 and I really liked it. It felt nimble, light, and the motor was not at all what I expected. I'd ridden a KTM 400EXC and its motor felt really lazy and soft to me, compared to my two stroke. But the 510 was nice and crisp, and had a quick rev to it.
The owner also has a 610 but he says it weighs about 100 lbs more and when he rode it off-road he just about freaked because he couldn't turn it LOL. He loves the 510 though, and I agreed that it was top notch. He doesn't have the dual sport stuff on it though. I'm trying to convince him to get it.
He did say that his 510 was boiling over when riding slow, and I noticed it didn't have a fan or a coolant overflow. I wonder if they add those to the street legal versions.
Hmm I just looked it up and there are two 510's. Now I wonder if the TE510 with six gears will be as sweet, or if it would be underpowered compared to the TC510.