I've had mine 2 years now (at least) and it's a Nike brand bag. Its got a decent sized main pouch that fits a couple textbooks and a notepad or two and a pocket that holds a 15" laptop. It's also got 2 side mesh pockets and a front pocket that I stuff writing material, computer cables, and so on in there. I'm actually not too sure what's in there... but it seems like Ragu, is in there! The bag also has relatively comfortable shoulder straps but I rarely use them. The only downside I've found is that the wheels are pretty hard, so it creates a lot of noise when I'm walking on anything except completely smooth sidewalk. I'm pretty sure that my backpack is this one: Amazon.com: Nike Accessories Rolling Laptop Backpack (Black): Clothing
I think we paid less than what is quoted there, but then again, that was 2 years ago and I really don't remember what I paid for it. What I can definitely say is that it has performed quite well since I got it!
Oh, and it really has been a back-saver. You certainly don't have to get one like mine. Many of my classmates use other bags and backpacks. Only a couple use a roller. Whatever you do, get something that's well made. You'll spend more in replacing junk with more junk if you don't. Most of the time all you'll need is something that can carry a textbook, a binder, some pens and pencils, and perhaps a snack or lunch. Nothing fancy...
When going to clinicals, I use a much smaller, lighter backpack that I've had for YEARS. I just use it to carry lunch/snacks, a couple reference books, and the paperwork I need for the day. That one, fully loaded, probably weighs in at less than 5 pounds.