Yeah it was on a trek soho, so more of a commuter than a mountain bike...i just assumed the rear sprocket which was attached to the shimano hub was all shimanos doing.
Anyway the nexus hub was heavy (not really a problem on a commuter) the belt also started slipping mainly because of the rear sprocket being worn...just generally not very good.
That said i loved the idea, the way it was sold to me as maintenance free cycling and a belt that would last for stupid amounts of miles...all seemed like a very nice idea.
I can see why shimano wouldn't recommend it for off roading, i mean just the slightest touch of snow on the belt caused it to slip and come off...what should have taken me 15 minutes took me more like an hour, in the end i abandoned the bike chained to a railing and walked the rest.
I thought i was alone but i read a post by a canadian guy who said his bike became unusable for 5 months a year as soon as it started snowing.
If it can't cope with snow i wouldn't fancy its chances of handling off road mud and crud...I may be completely wrong though.