I want a Toocan some day (the one made from recycled materials).
I have one pannier on my commuter. It's a Basil bag that converts to a shoulder bag. I never notice the weight of one- even when I have it loaded down. I don't ever feel off-balance or lopsided.. I think you'll LOVE getting the weight off your back. I rode with a messenger bag for about a month and when I got the pannier(s), I fell in love with them! I'm cooler, my bike doesn't feel any different, and I can carry heavier things without noticing the weight like I would on my back.
Now, when I grocery shop with the panniers, I do have to level out the weight or I feel lopsided (like with milk, OJ, canned goods, etc). That's the only time I can tell my bike is loaded is when I have heavy things. For normal work commuting- I can't tell a difference.