Does your messenger bag match your clothes/style? Much like all other men’s accessories your bag should complement your outfit. Here is my visual messenger bag guide to formality and capacity.
Messenger Bag Guide
The next most important question you can ask about your messenger bag is, what do you intend to do with it. To that end capacity is the second most important attribute. Unfortunately the larger messenger bags get the more casual they become. So if you are looking for a formal messenger bag and you need to haul a lot of stuff you may need to look to a different bag type.
Material plays a large role in formality as well. Synthetic fabrics tend to be more casual and canvas and leather tends to be more formal. There are exceptions. Tumi makes a line of synthetic bags that most would say are business appropriate and Saddleback makes some leather bags that are rugged enough to be casual.
My favorite chameleon is the Filson Medium Field Bag. It looks great for most occasions and compliments a suit as well as it compliments jeans and a polo. I managed to find an unusual version on sale at Nordstrom Rack.
I hope my messenger bag guide is of some use to you. Let me know in the comments and if you have more complex needs perhaps check out my bag database.