I actually bought this one after my laptop ran dead at a conference that had no where near enough AC outlets in most of the session rooms. I've used it exactly one time, and it is insanely heavy. Also, it never fully charged, even from day one (the charge LEDs are always 1 down no matter how long it charges). It's also noisy when you enable the inverter, and on inverter it's got just enough to recharge a 13" MacBook AIR.

And again, it's heavy. It weighs MORE than my MBA and charger combined, it gets really heavy in a shoulder bag, and even in a backpack it just ends up being unnecessary weight in must circumstances.

If you're often in situations where you're laptop is running dead, it can provide an emergency recharge, but at least for me, it's more trouble than it's worth and it just sits on a shelf now collecting dust. I wish I had never bought it, but it seems like a great idea at the time.