For the past 3 years I've been using an Ortleib Classic Messenger Backpack
View the Messenger Bag Classic at http://www.ortliebusa.com
The bag has been almost perfect for my needs. It holds a lot (30 liters, 1800 cubic inches), it's very comfortable,
it's also bright in color with reflectors, and is water proof.

However, the major negative with this bag is the Velcro enclosure which wears out fast,
which means the bag can open while I'm riding.
Once I was able to get the bag replaced on warranty, but other times I had to buy a new one (for $150)

I would like to consider other backpacks that are similar in size, comfortable and waterproof.
I would appreciate some recommendations.

P.S. I don't like panniers as they are difficult to carry around in pairs when I'm off my bike - urban walking, at coffee shops etc.