I am intrigued at the extent to which this thread has focused on the tactical type of day pack, which in my experience, aren't all that great. I bought a Maxpedition Vulture 2 a few years ago,drawn by its sturdy construction and adaptability (all those molle patches). Can't say I am too impressed, however. It is clunky and rather heavy, burdened with a bunch of features which only add to the weight, like load lifter straps which can't perform as such, but which look dreadfully cool. Its capacity is rather small,at 2800 cu. in.

If one is looking for a good, versatile, tough pack, seek out something that is designed for climbing and summit use. There are lots of packs available from brands like REI, LLBean, North Face, Kelty, Gregory, Patagonia,(among others) which will be well constructed and designed and will last for years of hard use (like the Camelback that Ben mentioned earlier). For a given capacity, they will be reasonably light. Don't let the lighter fabrics fool you. I have carried a Lowe "Alpine Summit Attack" (how's that for macho?)for many years and used it a lot. It features light weight fabric, and thin straps and outstanding durability.

Realistically, I don't think you will get a decent back pack for twenty bucks these days. Plan on spending more and doing the research and trials. You will be glad you did.