I started with a backpack, then went to a rack pack/trunk and for the vast majority of commutes the rack pack/trunk is plenty of space for clothes, lunch, etc.

On the rare occasions when I do need more space (esp. light/bunky items), I certainly have no problem riding with the back pack.

The rack pack/trunk stays on all the time, in fact it's bolted on, and it's mostly harmless even when there's nothing in it. Yes, it does come very close to heel strike, and in fact if I'm not properly clicked into my SPD's I can/do get a bit of heel strike. Nothing massive though and a gentle reminder (to self) to click in properly.

Rick / OCRR