Well Spa Cycles got the correct tyres (Marathon Cross) to me last week.
As it was their fault, they gave me a spare inner tube to cover my cost of return postage (my choice).
They went on fine (note: they are directional).
I went off for a 40 miler off-road this weekend and I found the tyres great (although I was blowing out my arse for the last 10 miles). Off-road is a lot harder than on-road - 40 miles on my road bike would have been a breeze, but this was hard work.
They roll really well on the road (I run them at 60psi), and 25mph on the flat is obtainable without much effort.
Off-road, they are also fine, even allowing wheelies under power on wet grass / mud - so plenty of grip.
As has already been pointed out, they are not brilliant in deep mud. You just have to be cautious and moderate your power.
But they handled everything else on my ride fine. This mostly consisted of grass / muddy bridleways strung together with back roads. They even handled a ride across a field of tates which some inconsiderate farmer had decided to plant over the bridleway.
The tread clears quite quickly as well when you get back on the road. You just have to be aware that you get splattered with the crap that flys off the wheels when the speed is high.
All in all, very happy with them.