This is a huge topic and I am not surprised that it hasn't been addressed on this site. There are a lot of variables involved, many of which are highly personal. There are also a lot of brands and many models available. These are constantly changing.<br><br>With internal frames, which is the most versatile and adaptable type to own (my opinion), a key factor is how well the pack fits. The only way to determine this is to try one on. This is best done in a good store with knowledgable sales people. This is especially important if you are looking at the larger packs, 4000 cu in and up.<br><br>I myself have around eight packs lying around the house, not counting my daughter's book packs. Each works best in particular situations, but I am always in the market for something just a little better.<br><br>In purchasing, be aware that many products have numerous bells and whistles that are designed more to appeal in the store than to provide real utility out in the woods. Generally these features just add weight, clutter, and expense. Keep it simple.<br><br>Check recent issues of Backpacker, Climbing, and Rock and Ice magazines for fairly objective evaluations of current offerings. Try renting before you buy.