busottiwrote a review of GoLite Jam 2 Pack - 3100cu in on April 6, 2009
First, don't buy this unless you are truly going light. I'd say 20 pounds. This will certainly hold more, but not as comfortably and the weight will not transfer to you hips as well as a pack with a frame (even noting that this has a hipbelt). If you are very experienced you can play around with the center of gravity and the manner in which you pack, but most folks find this easier to talk about than realistically achieve.
The fabric, nylon with dyneema reinforced grid, is fairly robust and water resistant. Great for lightweight 2-3 day trips, climbs or peak bagging. Longer stuff I would recommend an internal frame pack that will carry and transfer weight much better. If you are strong enough that you don't mind carrying extra load on your shoulders (instead of the hips), then you are probably strong enough to not have to worry about every ounce of pack weight. The GoLite Pinnacle model is a bigger version of this, but makes no sense to me. If you are going to carry that much stuff then you will want a frame. Don't make the mistake of fussing about every ounce of weight and then feeling sore all over, kinda defeats the whole purpose of enjoying the trip.