Despite it's lightweight and surprisingly durable (given the price) design, along with it's nifty magnetic closure mechanism, the JR Gear Dry Bag didn't really live up to its only intended purpose - to keep contents dry - when used in a variety of light water-based applications.
I've used the bag ritualistically since purchasing it approximately three years ago. The bag has gone kayaking with me, swam alongside me across lakes and has operated as my portable washing machine along numerous travels. In 2/3 of these above-stated uses, the bag let me down. Yet I still kept trying.
In all honesty, I had initially been shocked as to how water tended to enter the bag in the first place so easily. It carries an IPX4 rating, which isn't meant for anything longer than a brief submersion, but in even the most brief of submersions or the lightest of splashes, things become inexplicably wet inside.
If you intend to leave the bag on the floor of watercraft, don't expect it to stay dry. If you expect to be splashed from the movement of oars or by the whitecaps of waves, don't expect it to hold true to its claim. Like many dry sacks on the market, the bag's not actually waterproof, nor intended to be waterproof. Resistant, yes, but to an extent.
I've sat the bag bottom-down at the base of a kayak (which had some water swishing around) and the hoped that the rubber base would hold true. No such luck. Towing it along a lake with my personal possessions inside, the bag routinely gave way to water, though I understand that this is a rougher-than-usual use and hadn't expected a bone-dry situation inside anyway.
Specs as per JRGEAR.COM
30D Cordura® with silicon coating to increase the strength and durability
Taped seam for waterproofing
It could very well be through the taped seam work near the bottom or likely through the material itself, but this bag isn't much more than a deceptive splash guard.
On the bright side, the material is exceptionally durable, the roll-top is easy to use, and the bag is able to carry moderate weight without concern for rips or seam distortions. A bonus, the bag is easier to clean (thanks to the silicon coating) than most and stows away conveniently, adding very little weight in other packs.
The brightest spot about this bag is the magnetic closure mechanism, which incorporates a weak, flexible magnet into the roll top assembly. I found this feature to be particularly useful, especially when loading gear in a hurry or ensuring a tight roll-close.
Photo taken from JRGEAR.COM
This bag is to be taken at it's face value - it's ultra-light first and dry second. It's durable design is of great quality - I'd be hard-pressed to imagine a situation where the buckles and straps can breakdown. It's an affordable option in a market that sees similar water-resistance level bags being sold at much higher costs.
Though, with so many options available, this is probably one best to stay away from given it's questionable susceptibility to water. It's hard to imagine a dry-sack not even being completely water-resistant and, for this reason, its numerous pro's can't outweigh it's greatest con.