Just got back from doing 2400 miles over 13 days on a BMW GS F800 (15 reg) and my XTZ 660 Tenere. So you can't help a bit of a compare.
First thing is we never went off the tar and the XTZ had trail tyres and the BMW road based tyres. So this obviously clouded the opinions. But alas some opinions have been formed.
The 100cc bigger BMW def has more power and a lovely engine. It reminds me of an electric engine with little show of a power band. The XTZ in sharp contrast has loads of character and low down grunt especially suited to steep hills. The down side is some vibrations. In short the BMW is more refined and good on the motorway with a longer wheel base. The XTZ loves cornering with its short wheelbase. Fuel consumption was very close, with the XT perhaps 5% better. But when ridden hard there was nothing in it. The BMW seat is much better than even my Touretech. The low pipe, especially the cat section on the BMW is very low and vulnerable if you go off road, but this off course didn't concern us.
I thought the BMW twin engine would give me a clear indication of what a potential (rumoured) MT(07)Z would give us, but I'm still in mixed mind. For now I am more than happy with my XTZ. As a last remark, in 13 days I saw 5 other XTs, must have seen more than a 100 Beemers. Such a well kept secret.

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2010 XT660Z Blue, Exan can, UFO high front fender, SW Motech baseplate, heated grips, 12v point, Kevs Fuel & O2 mod, DNA Stage 2 Filter and cover, Givi tall screen, Mefo mfe 99s, Kevs throttle mod, TT high seat and 'compass'