My son went through 2 (cheap) backpacks his kindergarten year. The first one he completely destroyed. The second barely made it to the end of the school year.
Before he started first grade, I bought him an L.L. Bean backpack. Now he's halfway through second grade and he's still using it. It has fared very well; nothing has broken, no holes, hardly looks used.
It costs more than those you might find at discount department stores, but you get what you pay for.
I bought my daughter this backpack when she started middle school and she loved it. It was used through high school and as a senior in college, she STILL uses it daily! It washes well and has held up nicely! A GREAT buy!