I'd say they're not much better than regular jeans. Sure they're warmer, but if they get wet they'll take forever to dry, and weigh you down while they're giving you hypothermia. And when you're active, you'll overheat because the fleece lining isn't removeable. On the plus side, jeans are more durable than most synthetic pants, and the casual look doesn't stand out in an urban environment.
Your best bet for survival, at least outdoors in cold temperature, is the versatility of layering. Synthetic long underwear, synthetic pants(insulated for very cold temps or low activity levels), and an outer rain/windproof layer, preferably with full-lenght zippers for easy donning and removal.