They're good for short-term carrying around, especially in situations where stuff might get wet. But, I'd be very cautious about them for long-term use. As much as they keep water out, they can also trap moisture in.
The answer to that is often to throw some of those desicant packs in. Be careful. A couple of years ago, a forum member who was going on an extended trip packed all of his high-dollar knives into ammo boxes, tossed in some of those desicant pack, sealed up, and put 'em in storage. When he got back, his entire collection was hopelessly rusted. The conclusion was that the cause was... the desicant packs. They attract and store water. Under the right conditions, especially if temperature drops, they can release that water. To use them properly, you must first bake them for sixteen hours to remove all water from them and then put them quickly into the container you want to protect. But, be carefull here too since they can dry out wood, ivory, and other such materials.
Pelican cases are durable and water-tight. Great for carrying around, but not for long-term storage.