I had a 2002 and used it extensively on off-road trips. I had really good times on that bike and I still rate it as the most complete package out the box. (I do not have any experience with the XT660Ten)
The problems I had was all possibly related to the vibration of that motor. Rev that baby on the centerstand and it runs around the garage like a rabbit.
When I have to do distance on tar, I like to be able to at least maintain a steady 140km/h. This unfortunately is exactly where the bike vibrates worst, so much so that I have to ride with my legs open to keep them off the tank and your hands loose all feeling. I hated it on tar.
The hooter bracket vibrated in half, the needle retainer washer vibrated away in the carb, I had 4 batteries go 'poof' in short succession, the float level continually goes out of spec. Even SW Motech gave up on the protection bars because they would just crack due to vibrations.
Some of the batteries going poof may have been due to wear in the starter and too great a load, with the lights that stay on during cranking.
Fuel consumption was pretty steady at 16-17km/l. Irritating when your mate on his clapped out Dakar consistantly gets 24km/l.
In my group of riding buddies there were 3 640's. Two blew their motors, a 2003 and a 2005.
That said, I am considering getting another one.
It has everything an adventure bike should have. Large tank, proper subframe, wind protection, good quality parts, carrier rack, light weight, good suspension and easy to use.
The weight, height and general ergo's makes it a pleasure to use in seriously technical terrain. And at speed over rough terrain it is sublime.
I have spent a lot of quality time on that bike and I have a very soft spot in my hart for it.