X6 is an excellent car, i had one for this seasons regionals (now sold, but only for cash flow reasons). Very strong, and uses the same gearbox internals as the b4 ft, (not b4.1, that uses metric bearings for the diff).
It is currently on a slight go slow in development terms though due to xfactory currently developing their 4wd buggy, the x7, but I am sure they will be back on it again soon!
However I would go for a second hand one if possible, as to buy new, they are quite expensive, but do include a lot of nice parts such as titanium ballstuds, hinge pins etc.. A real top spec kit.
The x6 has seemingly lost quite a few club drivers to the losi 22, but they seem to be struggling a bit, and are missing their x6's!!
The geardiff seems a bit hit and miss. Most of the problems come from installing the shims that keep the diff mesh correct as part of the rtr build (not 100% sure on this however). However, I would confirm the situation on this with the UK importer - CML.
Hope this helps.
M.