I got the Chrome Industries Sentinel model because it was the perfect size and had enough features and pockets. It's on the smaller end of backpacks, but holds enough for a day's commute--I specifically didn't want something so large I could fill it with too much stuff. Flapped, not roll-top, because I valued ease of access over ultimate waterproofness. Only downside is the squared off top, which interferes slightly with peripheral vision in situations when I'm looking over my right shoulder, scanning traffic.