I've used Mixmasters a lot - in Antarctica to -40C, on Aconcagua in -25C and in Tibet up to 6000m. They're great pants, but you need to keep moving and have it be not too windy. At those temps you really notice that Powershield is not 100% windproof (80%). But MM, or Winter Guides, are certainly the warmest softshell around. Most would throw on some synth overpants (MH Chugach/Trans?, Pata Puffball etc) after about -20C.
I also use Cold Fusion softshell pants from www.beyondclothing.com
and they are usually too warm for summer and OK for light winter use.