I've been commuting to a business casual office for 5+ years. I keep a spare set of clothes at work, along with my shoes and belt. Each day I fold my work clothes inside my pants (folded in thirds), place that into my pannier or messenger bag, then put my lunch box in the bag. I've used a Chrome Citizen messenger bag most days. Recently I've been using a single rear pannier more often. An Ortleib Back Roller would be perfect for my clothes, lunch, and bike tool kit. However, I use a Jandd Mountain pannier with a 20L dry bag inside (because that's what I've got). The messenger bag keeps things organized and non-wrinkled, and it's easy to get everything in and out. The down side of the messenger bag is a sweaty back and my eyeglass mirror can't see past it. The pannier is a little slower to get things into and out of, but the bike ride is more pleasant.