The XT660Z 2008- is basically a very heavy bike for its size. A KTM 690 is 42 kilos lighter and a Suzuki Dr 650 is 35 kilos lighter.
And the big single engine is restricted and lumpy, is surging etc and has a narrow useful rev range.

I rode one XT660Z from Norway to Australia - approx 85 k kms and I have just turned around and will ride it back home now.

The engine is quite reliable and the frame are strong so thats good stuff.
The cush rubber drive in the rear hubb wears out quite fast - maybe only 15 k kms or so. Its useful to mod it by packing it with inner tube rubber or make small metal plates that fits inbetween the cush rubber and the hubb.
The rectifier is also a weak spot and many XT660Z owners have had to change those. Mine has however been fine so far.
The gearing is set up very high on stock bikes - cruising at 100 km/h the rpm is 4000. That is less rpm at 100 km/h than much bigger bikes. The stock chain/sprocket size is 520 so its imho a bit underdimensioned for such a big bike so I have put on 525.

And then the usual - suspension isnt top class, seat, handlebars, footpegs, headlights etc could use some improvements. But I guess that goes for many stock bikes.
And as mentioned above - engine is terribly restricted, consider new airfilter, exhaust and fuelling program improvements.

The XT660R is actually 20 kilos lighter I belive and by so a better choice. But it has a much smaller gas tank.

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In the end everything will be fine. If its not fine its not the end....