Well, FWIW, I just traded my KLX250S in on a Duke!
I pick the Duke up in a few days.
My KLX was only a couple of months old but I realised that most of my riding would be on the road, not the dirt. And you are right - the KLX is not the greatest on the highway, keeps slowing down on hills.
The Duke only weighs a little more than the KLX but has three times the power, so you do the maths

. Sitting on it, it is more comfortable than the KLX (despite complaints on here about lack of seat padding) and doesn't feel remarkably different to the KLX in terms of posture. The seat is an inch lower than the KLX so is easy to mount. The Duke certainly exudes quality and looks even better in real life than in photos! I can't wait.
So, besides the looks, I'm really looking forward to the bike's excellent handling

. I live in the Adelaide hills (South Australia) - lots and lots of twisties - twistie heaven. So I'll just be *forced* to ride up and down the winding roads where I live *every day* on the way to work and back
Then on weekends, I'll again be forced to ride through yet *more* twisties to get to a southern coastal resort to visit friends. Aaah. It's going to be a tough life

.