Messenger bags are nice because you can typically access the contents without taking them off. They can also handle large and odd sized items.

Just from observation, it seems that riding style and bike geometry have a significant impact on whether one prefers a backpack, messenger bag or panniers.

I've used all 3 but have settled on a backpack for now. My commute is intended to be a workout so I sweat backpack or not. I will also make a stop or two along the way so I find a backpack more convenient. Don't particularly like the way my bike performs with a loaded pannier on it either.

Folks on a more upright bike will find a backpack more uncomfortable than people on a bike with an aggressive geometry. The latter spreads the weight more evenly across back and shoulders.