I was thinking of some panniers for my 10 mile offroad commute (very rocky in places), but following some advice on here I went for the Carradice SQR Tour saddlebag. I can’t recommend it enough, I don’t even notice it while riding. It’s only 16 litres in capacity but I fit the following in mine:
Clothes for the day
Change of shorts and top for the ride back
Waterproof (I have to get on the train and wearing the one that I cycle in might upset my fellow passengers)
iPad
Mini-pump
Spare tube and tools (in side pocket)
Magic shine light (other side pocket)
I couldn’t fit shoes in but I just leave some at work and change when I get there. Occasionally I need to bring my laptop home so on those days I use a backpack for just the laptop – much more comfortable than carrying everything in the backpack, and stops the laptop getting rattled around as much.