My main laptop bag is a SwissGear GA-7305-14F00 15.4 Gray Notebook Backpack. I've been using this backpack for years and what I like the most is its virtual indestructibility. After logging thousands of miles with this bag in trains, planes, and automobiles, I have yet to get so much as a scratch on any of my laptops. The bag has been dropped several times, knocked around in crowded subway stations, and treated carelessly by flight attendants in overhead compartments. With all of that abuse, something bad should have happened by now - but it hasn't!
Another nice thing about this bag is the multiple compartments which allow me to carry everything I need, including even a water bottle. At times I carry both my main laptop and my netbook, and this bag handles double-duty well. I have no complaints whatsoever. The price may seem a little steep, but it is well worth it.