They are absolutely fantastic on dry hardpack mud, roll quick but still have good traction and braking, they're pretty good on dry rock too, in mild damp soil they are tolerable, the edges still bite but the centre tread can slip a bit. However, show them wet roots or wet rock and they WILL kill you, mercilessly and without warning, normally by ejecting your bike sideways from underneath you at just the wrong moment!
Basically I like them quite a bit, and still ride them in the wet, just with a fair degree of caution on anything rooty!