About two weeks ago I got an Ortlieb Messenger Backpack. I still have a custom Under the Weather Heavy Gear Mess bag and before that I had a Chrome Metropolis (the Chrome had to go because the UTW was a gift -- which I cherish). I enjoy the UTW greatly, however, my separated collarbone (grade III and healed but the bone sticks out and still gets sore occasionally) wasn't dealing too well with the mess bag over longer days and longer distances (basically all day riding or being out and about).
The solution for me was a backpack for the various reasons stated above: even and better weight distribution, very comfortable when off the bike and walking around and because I'm not a messenger and have no need to pull things out of my bag very often, don't need that kind of quick accessibility.
The Ortlieb's have always attracted me with their clean lines and so has the Chrome Backbone but I had one when I first separated my collarbone and the straps were a little too close together at the top and rubbed my neck the wrong way and the straps ended up being wide and put pressure on my collarbone.
A friend had an Ortlieb I got to try out and I was impressed with their minimalist design and simple closure -- a roll top waterproof bag that had a simple velcro strap up top. The padding that comes on the back has grooves that actually do facilitate air getting between your back and the bag and feels good when you're riding. A thin flexible plastic sheet inside (with elastic bungee cords) helps the bag keeps it's rectangular shape. It has both a horizontal chest and waist strap allowing the bag to get really secure but I've had no issues riding with just the chest strap clipped.
It holds a lot of stuff -- and because it's relatively rigid, is perfect for piling from bottom to top, stuff that you'd have to arrange carefully in a mess bag to feel comfortable when you ride. It's actually quite impressive at how much it holds.
It's a remarkably comfortable bag. Long days and long rides with heavy and non-heavy loads have thus far put it to the test comfort-wise and it's come out quite well and admittedly a better bag for me while cycling.
So far, so good.