They have the best prices I've found on Cylalume sticks, signal mirrors, compasses and other like items. Service is prompt. They run adds in American Survival Guide frequently. They do not have a huge variety, or a fancy catalog. They concentrate on specific areas and do a good job in those areas.
SI is probably the largest survival mail order supplier around.
They have the widest selection of products of any single supplier. Their catalogs are monthly 32 page two color magazines that only cover a portion of their product lines. Most items have a paragraph extolling the virtues of the item. Please note however that most items can be purchased elsewhere for slightly lower prices IF you don't mind spending many hours looking through dozens of other catalogs. Buying many items from one location with slightly higher prices will sometimes be cheaper than multiple purchases from multiple places due to shipping charges savings.