I've got the Osprey Radial. It's a little bigger than I need on most days, but does have capacity for those rare occasions when I need to carry something extra. It has a tucked-away rain cover, but that mostly seems to be to keep the rest of the pack clean—water hasn't intruded even when I don't use it. Seems to be good quality and has held up well. It's pretty well thought out for commuting, although I did add some velcro loops to the "tails" on the shoulder straps to keep them from flapping around.

I've got a locker at my workplace, and I hang the cycling gear I commute in there (also a towel, toiletries, and the shoes I wear at the office). So my packing tactics are probably different than yours.