I use an Osprey Momentum 26 while commuting on a carbon drop bar roadie. I've been through a couple of Chrome bags - and a pair of shoes as well - but find their pricing is not commensurate with the average performance and build quality.

On Mondays I pack:

Scrubs
Lunch for the week (1lb deli meat, deli sliced cheese, tortillas, frozen burritos, mustard)
Shoes
1 liter Thermos of yerba mate
Tube and a flat kit
Cards, money, phone

Lately the weather has been volatile as we transition to spring, so often I also have cycling socks and a jersey to switch out for the ride back. The pack comes with a rain cover which works great, but for unexpected precipitation I wrap all of my junk up in plastic bags. I'm starting another degree later this month and will be lugging all of the above plus textbooks and will have no problems with moisture or cargo room. It's not a cheap backpack, but it will last many years.