Arkel and Jannd make some off-road specific panniers, I think they are a bit more low profile with regard to hanging up on vegetation, etc. Apart from that, I would check out Ortlieb and Lone Peak (I second the recommendation for The Touring Store). Watch out for big handlebar bags, they can really change the handling of the bike and that will be important off road.

The Lone Peak bags are great, and their mounting system is totally fine for road touring, but I would question them for off road touring, you might want to add a back-up clip so they don't bounce off the rack. I hate saying this, b/c they are my favorite bags. Arkel and Ortlieb attachments are bomb-proof. Arkel stuff tends to be overly heavy.