Its called the Viscacha by Revelate Designs. I can stuff a change of clothes in it, a cable lock (I keep a U lock at work), and there's room for more. It expands so you can get a good bit in it if you had to. It's fairly secure - not swaying all over the place. It's not waterproof so I keep clothes in a quality plastic bag. The frame bag is also from Revelate. I store my repair kit, phone, food, money, etc in that. Again, not waterproof but it hasn't been a problem.

I think Revelate (and other brands) have a similar dry bag system. It works well for me. I'm a video editor and I work in a cave so fancy dress isn't required. I roll up some shorts or pants and a fresh shirt and I'm good to go.

The only time I really notice the load is in high winds. It's a bit more work strapping these bags on the bike but once it's on I'm riding comfortably.