Great bag and I've bought a couple more now so I don't have to move it across bikes. It is spacious, waterproof, and has a truly durable zipper (unlike so many others). It can move around a little bit if you don't cinch it down sufficiently, and can bump the knees depending on your Q factor and riding style. Those nitpicks aside, I prefer this bag over many alternatives I've tried, and that is a long list!
For the other reviewer with the quality issue. I own quite a few Ortlieb bags by now, and all have been good quality with one exception on a different style bag. For that case Ortlieb handled the replacement expediently.