I bought this commuter bag because I was tired of my other bag flying off my rack whenever I hit a bad pothole. It is so nice to not have to worry about that anymore. Also, my commute involves three bike rides separated by a trip on a ferry and a trip in a train. So I am constantly taking my bag off the rack and putting it back on. This bag is just perfect for that. The Ortlieb bracket design is awesome. It was easy to install on my bike, feels secure and sturdy while not adding much weight, and makes the back of the bag smooth and neat, without anything that catches on your clothes or gets caught on other things. The one drawback is that you can only use the bag on one bike, although Ortlieb probably sells these rack adapters separately if you want to install two or more.
Aside from the connection to the bike, other things I love about this bag are the size (not to small and not to large), the look (fancy but not tacky), and the ways you can carry it while walking (messenger style and like a briefcase in your hand). Also, the interior organization is both convenient for laptops and other business stuff and flexible enough for larger things like clothes or lunch.
The only thing I have not yet experienced is how well this bag will hold up in the rain, since we are in the dry season now here in California. I will update this review to reflect these experiences after it starts raining again.