I have a 1988 NX650, nicely farkled I might add.
I do like it quiet a bit. I would agree being a dual sport if is good at almost everything and great at none. I think that "Merle O'Broham" described it best. I tried the taller home made screen for a while and it did help, but not as much as I would have liked, so I am back to stock. You do have to hang on a bit at freeway speeds. In cross winds on the freeway, it is a little un-nerveing, but I would assume any middleweight (640A, KLR, F650GS) would have similar problems with getting moved around a bit. I call it my perfect 50mph bike as up to that speed, I think it is FANTASTIC. Off road it is a touch heavy and I too have bottoming issues over big jumps. A new rear spring properly sprung could fix that problem though. After all, the US versions are almost 20 years old and the springs should be a little tired by now.
If you can pick one up around the $2K range, I would encourage anyone to do so, they are good little ride and very well put together.
A little history for those that do not know. In the US they only sold the NX650 for 2 years 88 & 89. To my knowlege there was no 650 dualsport offering from honda from 90 through 92 they came back with the more offroad orentated XR650L in 93. They did have the much loved Transalp during that void 89-92 (i think) but that was a 600 twin.
Oh and the Acerbis 22L tank upgrade, Yeah that is a big plus, now over 200Miles on a tank!
Hope this has helped