I cannot comment on that particular brand but I have two custom made bags that I really like when I have to carry something. The easy on and off is just one feature that I like. Both have clips in the middle so that it comes off like a knapsack. The sling over your shoulder to get to what inside is certainly why couriers use these bags over panniers.
Panniers to me are the absolute worst to carry something on your bike unless you are camping and need all that stuff. The problem with panniers is that it feels like the "tail wagging the dog". The bike becomes this heavy weirdly weighted apparatus instead of the fine sleek machine it started with. To me, it is all about the bike. How does it ride, how does it track, the agility of the bike. Once you put on panniers all that is gone. Now it rides straight really well and that's about it. Don't get me started on front panniers there even worse.
I rode with rear panniers for years, I rode with front panniers, upper and low rider varieties, I rode with cycling knapsacks (which is my current favorite) and I have rode with messenger bags. All my messenger bags are custom made from PAC Design and from Push the Envelope. I have also owned aTimbuk2 bag as a backup but not in the same league as the others. I sold off two other bags last year since I was drowning in bags.
Bottom line, messenger bags are great, don't rule them out.