Just tried the Jam last weekend on a day hike with the back panel removed and a 3/4 ridgerest rolled, loaded to 24 pounds to simulate a week long trip, carried okay for 12 miles but I thought the shoulder straps were too narrow and the hip belt seemed to slide down alot. I need to give it more time (this weekends trip didn't work out), but I may go the ULA Ohm route instead. I do like the side pockets, back pocket, and hip belt pockets while on the trail though.
Like Serge said maybe try two or three packs, keep them clean, don't cut on them, then return what you don't like. You can get the Jam at REI, 20% off ends today.