There have been some great reviews of the Imago in the forum, but I have to mention how pleased I was with this bag on a recent weekend birding trip. I couldn't wait for the small version of the brain bag and opted for the messenger style bag instead of a backpack for birding. And it worked great!
My Treo fit in one of the small side pockets and a 4 oz Amphipod with water fit in the other other side pocket. Access to my bird book, binoculars, and jacket was easy while walking with the rest of the birders. As it says on the ordering page the "waist strap secures the bag to your body when you're the move". This was especially appreciated as we skirted poison oak along the trails.
One morning it rained on us for about 3 hours. Everything in my Ballistic nylon Imago stayed dry and the cork trim is as handsome and unphased as if it hadn't felt the rain at all. (I knew rain was forecast but figured if cork can hold up for years in wine that a little water wouldn't hurt it. Fortunately that was true.)
Even though I opted for the comfortable Absolute strap, having the waist strap on helps to take some of the weight off the shoulder.
The Imago, another great TB bag, has become a great trip companion. (I chose Hunter to match my Aeronaut - nice set!!)
I'd actually like a second Imago in a smaller size I could use for an everyday purse.