I love this bag. It's completely air-tight. When you close the zip it's like a ballon - you squeeze it but no air comes out which means no water can get in.
The reason it's a 4* rather than 5* review is a minor gripe. There are no end handles and the handles it has don't feel particularly comfortable. However, they minimise handles to minimise the number of welds and possible leaks so it's understandable.
Overall, a great bit of kit if you want to keep your stuff 100% dry.