I usually have both a Carradice Nelson saddlebag and an Acorn Boxy Rando bag on my commuter. Neither are well-suited if you need to carry a bag around with you at work - I usually don't, and also am able to bring my bike into my office so don't have to worry about taking bags on and off. I love the front rando bag - keeps stuff like cell phones, sunglasses, sweat rags for hot days easily accessible. And it's a cinch to take on and off, just not a bag I like to use to carry stuff around. Sometimes I'll just strap a briefcase on the front rack - and once I do some rehab work on my single speed, that bike will have a front porteur-style rack to facilitate easy carrying of a briefcase or other non-bike specific bag. Carradice saddlebags are wonderful but even with the quick-release options are not sensible bags to carry around with you during the workday.

Standard rando bag designs (like what Acorn and Swift use) aren't totally waterproof - not because of the materials, but because of how the opening is designed. Easy enough to carry your things in a waterproof stuff sack or a plastic bag though.