Was blown away with my trailrakers, went for these on several recommendations and they grip like nothing else on grass, mud, roots even tarmac! The 2.1s are OK, even on a mavic 729 DH rim – I am tough on tyres, ride a hardtail and hit rocky stuff whenever poss and the trailrakers have been fine (usually run 2.3 rampages).
I rode a boggy climb with about 2" of sludge on top of roots, it was one of the steepest climbs I had ever done and the tyre only clogged a little, aimed for the roots and it just gripped like glue… never got this amount of traction from any other tyre and i have used loads.
I would say they are almost lethal on 'surfaced' tracks i.e. loose stones over hardpack.