Although the idea of a personal flying machine small enough to fit in a backpack is extremely appealing, you must consider the following:
1) Wed have to wear aluminum pants, which are probably not really comfy.
2) Engine failure at 75 feet in the air is not cool.
3) To be useful, a JetPack would have to go fairly fast. At that speed, birds, tree tops and clothes lines come mighty quickly.
4) People would use them as safely as they use boats or ATV cluless and drunk. Imagine a group of drunken fratboys with their JetPack on a windy day in the busy downtown core…