That seems like a nice option if you have a rack. It's always nicer to not have the load on your back if you can manage it.

I don't have a rack on my bikes, and use an Ortlieb Messenger Bag as my commuting bag. Used to have the smaller Velocity model, but it was often too small for everything I needed for work: a change of clothes, shoes, lunch, laptop, etc...The Messenger Bag size is just fine, though.