During the last PC fair, many booths promotes powerpack because today's mobile gadget are really power hungry. Even though the specifications mention that the battery on your gadget can last for days, most of them actually can't even last for a day due to the push function of the gadget and accessing the internet. For example a Samsung SIII has a battery standby life of 790 hours running 3G but will run out of juice less than 24 hours.
So the extra power bank will be a must add-on to your gadget bag so you will not run out of juice at the time when you need it most.
A 5000 mAh Mipow power bank, able to full charge a tablet
Samsung Power Pack for Tablet 10.1 inch