We live out of them, we carry them, we pack them and unpack them every day, they hold our snacks. Bags are the second most important gear of our cycling trips, besides our bikes.
Travis and I carry two very different style panniers. He carries most of our stuff (thanks Babe), and uses Jandd panniers that can expand to almost twice their size, and have all kinds of bungees and clips to attach gear to. His bags aren't waterproof, but he has waterproof covers that work well. I use Ortlieb top roll bags, which are waterproof but non-expandable. I really like them for tour and for everyday commuting. Neat accessory: the Ortlieb Pannier Carry System. These straps connect to the pannier and convert it into a backpack. This is awesome for nights when you don't have a campsite and have to leave your bikes at the road and carry all your camping stuff up into the woods.
Anhaica Bag Works
Our sponsor Anhaica Bag Works designed the "Everything is Tourrible" bag just for us! It can be used as a hip pack or messenger bag. Travis uses it for his camera equipment, and I like it for the times when I don't need a full pannier to carry just a few things. Pro tip: it fits a six pack.
Travis had a randonneur bag during our Arches to Dunes tour, and I didn't. It was awful. Travis had all the snacks, easy access to his wallet, a place for his bike speakers. Well I got one this year. It needs a decaleur brace to steady it on the rack, and Velo Orange makes those. They are large enough to carry a decent amount of items, and accessible enough to reach in and grab what you need without getting off the bike. Check back with Anhaica's site every now and then to see if she has some in stock, or email firstname.lastname@example.org to see if Marina can whip one up for you.
The bicycle accessory bag is so versatile can be used for anything. There are plenty of small items on tour that need to be kept together- snacks, electronics, toiletries, keys phone wallet, tools. And this little bag is waterproof, so feel free to accidentally spill your toothpaste in it.
Remember, panniers are just big open, bags and are difficult to organize without help. I bring little bags to section off my gear-- think of them as drawers in a dresser. All my clothes go in one small tote bag; my socks, bras, and underwear in another; small pouch for condiment packs from the gas station; big mesh bag for food. This way you can find what you need quickly without having to take every single thing out of your pannier. One day when I am fully sponsored and don't have to buy gear, I'd like to have stuff sacks for my gear, but for now I use small tote bags, zippered pouches, cloth drawstring backpacks, and whatever else I've collected throughout the year.