I feel that it's all about the environment that you'll be working in, as well as the field. My wife is an accountant and a backpack would never be acceptable to her environs. In my little patch of IT, a backpack is completely acceptable for just about everything except meeting with a CEO. (Although, I have met a couple Fortune 50 CEO's and they didn't seem too terribly concerned that I had a nice backpack vs a leather bag.Then again, I'm the customer!)