OK, I don't have one of these. If I didn't already have a pair of Ortlieb Mini Biker Lites, an Arkel Utility Basket Pannier, and a Nashbar Briefcase Pannier, I would jump on it right now!

Here are my observations about balance, which confirm Arkel's notes on balance with regard to the UBP. I have loaded up the UBP, which is a single huge bag, with some pretty heavy loads and never even notice it. When you are riding you will never notice a heavy unbalanced load because your body automatically compensates just as it does if you carry a heavy suitcase in one hand. Just don't try to put the bike on a kickstand with a heavy load on one side. Your bike is not as smart as your body. At least most people's bikes aren't.