I have 3 bags. A Timbuk2 Messenger bag, a Chrome Messenger Rolltop Backpack, and a Banjo Bros.
Messenger Backpack.

They all keep everything dry. I use the Timbuk2 for city riding/commutes so I look urban chic , but it shifts around a lot more than the backpacks. It is comfortable, just shifts around more.

The Chrome is very comfortable and huge, so I use it when I need to pack a lot. it works great.

The Banjo Bros. is probably the best all around solution for me. Not too big, comfortable, and I use it
probably 60-70% of the time.

I do prefer the backpacks over the messenger bag, mainly because I have a fairly long commute, and they are more stable for me without shifting around. I rarely have to get something out of the bags after
I start rolling, so that isn't too much of an issue for me, but the Messenger bag does work easier for that.