This might seem obvious to most but I just realized these days how important the battery is for the mobile experience. I recently bought a netbook (see here and here) and while most experiences are positive, a battery lifetime of only 2 hours just doesn't do for me in many cases especially when I am traveling. Even if it is enough, connecting the netbook back to the mains all the time for recharging is also a hassle. So I bought an extension battery pack which gives me 6 hours of autonomy in addition to the 2 hours of the standard battery. An incomparable experience! Now even while traveling for a whole day, sitting in the train, waiting at the airport and on the plane, I don't have to worry about the netbook running out of power. Very nice!