Several riders I know have not had good luck with M-12 rears. I've never tried M-12s, but I found the S-12's to be a great soft-terrain tires. Unfortunately the S-12s didn't "hook-up" well on hardpack unless running really low pressure, ~8psi. The rear will chunk knobs in rocks and the angled knob on the front affects braking traction on hardpack.
On the rear I've been pleasantly suprised by the IRC VE33. Up front I've had good luck with a Bridgestone M59 and mixed results with a Dunlop D755. The 755 needs lower pressure, <10psi, to work as good as the M59. You do run the risk of pinch flats by using low tire pressures in rocks.