I bought this to replace my Osprey Map Wrap which is waterproof material but I found the stitching allows water to seep in under a true downpour.
These just passed the shower test and appear to be bomb proof. So compared to the Osprey...
The Osprey is perfect for strapping to your shoulder strap and keeping as a quick reference.
The Sealines can be rolled and strapped in a similar but less convenient manner. I'm considering strapping on a lanyard and velcro to get the convenience of the Osprey (I really loved that thing till it leaked).
Sealines are more versatile. You can strap it to your boat deck, your shoulder strap, the back of your buddy's pack. It also allows you to view both sides of that map while the Osprey only has one side as see through.
Most important though, these are REALLY waterproof even without seam sealing.
Only lost one star because I've been spoiled by the Osprey shoulder strap attachment and this doesn't have that feature.