I am a long time fan of SpecOps Brand products. Their stuff wears like iron. I have two of their T.H.E. Packs, one of which I have carried all over the world for going on 8 years now and other than a broken Fastex buckle (which they fixed for free), I have had no problems with it regardless of what I have loaded in it.
These packs usually go for $200 bucks and to me, they are well worth the price.
While I was home in the US last time, I bought one in desert camo for $99 (they are selling them out) and brought it back to Egypt to use as a sand backpack for exercise. I have it loaded with about 35 lbs of sand now and it rides pretty well, I'll tromp around the job site for an hour or more with it and it does fine.
I realize that even $99 bucks sounds like a lot but it is an investment that pays off IMHO. I feel sure that it could handle another 30 lbs of sand without a problem, just don't think I could....yet.
And when you are not carrying it around on your back, you can use it for some sandbag workouts and, I suppose, even carry other stuff in it around town.
T.H.E. Pack, 3-Day Assault Pack, 3-Color Desert Camo