For sommuting I have been using a backpack since day one and find it fine. I can easily hope on and off my bike with it for multiple stops, and easily fit in a change of clothes, a lunch, a binder with work items, and more if I stop at a shop for instance. Plus numerous side pockets for tools, raingear, etc, that you never need to bother with till you need them. Don't worry about the summer, yeah it will get a little warm but youre going to sweat anyway if you are in high heat and humidity, but if it bothers you that bad, or if you have a superlong commute, then maybe its not right for you.