When I was a kid, I remembered buying at least one new bag a year. The el-cheapo stuff we got always tore open from the heavy books we carried to school, or just simply the crappy materials being worn out over time.
The first bag that lasted me a damned good long while, was an Osprey. One of my friends was a mountain climber - maybe that's the wrong word, but she would hike up to the Himalayan Basecamp, practice over at Mount Kinabalu over weekends.
Prior to one of these expeditions, she got an Osprey pack that she didn't need, and offered it to me at a good price - and I took it up.
Fast forward the 13 years+ with that Osprey, it was still working great, until some idiot left it in his London rental before leaving for Montreal >.>
That Osprey pack - I don't know what model it is - has been through loads with me. Sea, Snow, Storm. Holding my lenses and what not. When I got back to Singapore for a break, I did go check out the Osprey bags there, but the prices turned me off, and I decided to go for a daypack off one of them internet resellers. It wasn't particular cheap, nor was it super expensive (SGD30+, iirc). It did the job.
For 3 months. And then the bloody zipper came loose >.>
Get a good pack, it lasts a while.