I use a trunk rack, and two nashbar townie folding grocery panniers... I really like the set up. The trunk rack holds stuff like keys, change of clothes. The panniers unfold when I need to bring a bag of groceries to work for lunches, or the woman asks me to pick up a few things on the way home.

I often ride with just one side expanded, filled (1 bag of groceries). Have had no problem. Granted, it was not a lot of weight. If it were heavier, I would have divied it up between the two. Even with both expanded, it's pretty easy to weave through gridlock, and pass the slow cars.

Some people worry about how easy it would be to rip the panniers off, but I bring my bike inside at work (besides, they were a lot cheaper then regular panniers).

However, I would personally be nervous about plopping a laptop into one of my grocery panniers. I don't know if my fear is founded on reason or not.. I'm just giving my gut reaction.