I use two with a briefcase trunk bag if I'm carrying a laptop. One for jacket, arm warmers, extra tubes, etc, and the other for lunch, wallet, other stuff. Probably irrational, but I like the balanced approach. I keep everything in the bags - shirt gets too sweaty to keep stuff in my pockets, probably because the bike is too heavy with the bags :-)

I like the trunk bag for my laptop, it feels very secure and balanced.

One problem I have is that the laptop case is only compatible with panniers that have the hooks right at the top - any part of the pannier that sticks up above the hook interferes with the trunk bag. I am borrowing and old set of Trek panniers, and would like to buy a set. The Trek/Bontrager interstate model would do, but it's pricey. The Jandd economy set also looks like it will do, price is good, has anyone used them?