The only time I'd consider a backpack would be for a short and lightweight trip — possibly a long-weekend credit card tour with short daily distances. Even so, I'd look at ways to put the backpack onto the bike instead of on my back. Right now, I'll ride with a backpack when I'm going to work, but I have a very short commute and I keep the pack light, so it's not an issue.

Before going with the backpack, test it out with the load you'd carry for the distance you plan to ride in a day. At the end of the day, see how you feel. If you still plan to use the pack, then make sure to stop regularly, remove the pack and rest your back.