another bag fanatic here . . .
I have tried quite a few over the years. Right now, I have several I turn to depending on the length of the trip, and if it is business or pleasure.
Messenger style: I have had 4 Timbuk2s over the years. Great materials and just a super warranty! But there is one thing that bugs me. They have lots of nice individual compartments, but they sew them all flat. So if you put something in one of the zipper sections, you can't use the others. There is no depth to most of the pockets. In this regard, I would go with Waterfield messenger for pockets that are more useful.
Shoulder:
For longer trips this is a great carry on. Best strap ever! This is a stock photo,
go to redoxx.com for more. this is the color combo I have (safari)
Around town, here is a great option. Chrome Vega
For an in between, I have this one, that has a dedicated padded pocket for an iPad .. Hazard 4 Kato. Just a note--the padded iPad pocket is very snug if you have a case much thicker than a smart case.
For business, Tumi has some nice options. I have an old one of these I used to use for international travel, but works super for iPads. Tumi also realizes that the things you want to put in pockets (sunglasses, chargers, etc) have depth, so they make the bag really useful.
Next up? thinking of a sling to take weight off the shoulder a little. I figure I am OK as long as I have fewer bags than my wife has purses. . .