I second that. However, if you also want to use it as a messenger bag, I prefer my Vaude Bayreuth II, since its hook mechanism can be closed.

With my Ortlieb bag, I regularily get stuck with one of the hooks in the back pockets of my trousers, so I have a habit of always removing the hooks altogether if I'm not using my bike, which is a bit of an annoyance. The Ortlieb bag is much more durable however.