Haven't ridden the F800GS, but I have rode the F650GS (same engine, different tuning) as the ex Girlfriend has one.

I will be honest and say that, assuming the F800GS rides like the F650GS does then, if I had ridden the F650GS before I bought my Tenere I would have bought the F800GS instead.

Don't get me wrong, I love the Tenere and think it's a great bike, but the engine in the BMW is just so much nicer than the yams. That said I would have soon got annoyed at the prices BMW charge for, well pretty much everything. In addition the Tenere is a lot easier to do your own work on.

One final thing, the BMW is very good on fuel consumption, so unless you are planning on travelling in some seriously out of the way places I wouldn't be put off by the smaller tank.