I know this isn't technically a jetpack, being that its mounted to my feet and all, but it still counts in my mind. This was a crazy fun 1 year anniversary gift my wife got for me (30 minute session for $100 via Groupon in Virginia Beach).
The concept is simple, I take a board and strap my feet into the mounted boots. The board itself gets its thrust from the attached jet-ski. So it becomes a follow around device that has no control over where it goes, only how much thrust is given. This is operated by the guys who are selling the timeslots on this device and they make sure to keep the rider safe.
If you get the opportunity to do this, I highly recommend giving it a try. It is very fun and a great adrenalin rush! See some of the pictures my wife took:
I'm no pro, but if you have any questions on this feel free to ask! I'll share what I encountered in my first attempt at this sport.