At 6.7 millimeters the Jumpr Slate is technically a little thicker than the Bookmark, but that's across the Slate's entire surface. The Bookmark actually gets pretty thick on one edge near its USB ports. And the Slate's battery capacity is massive in comparison.
Of course you can't magically squeeze a 10,000 mAh battery into a smaller package, but by spreading it out into a large, thin enclosure, EnerPlex has created a backup that's easy to slip into a documents pocket in your bag. So as far as form factor goes, this is a nice alternative to a massive brick.
It's also nice that EnerPlex has included two regular sized USB ports in addition to a microUSB port for charging smaller electronics. And the price isn't bad either. The 10,000 mAh version is just $100, while a smaller 5,500 mAh version is even cheaper at $60—but do yourself a version and spring for the heftier model. More here.