I remember seeing those before. If I recall correctly, they made several versions of it. Johnny Quest s features those things in it's intros and in one or more episodes.
It seemed like they should have been pretty useful, but the military panned the idea for some reason.
The advantage of the jet-pack over one of those would seem to be stability and security against being separated from your flying machine. It looks like you can easily fall out of one of those flying pads.
I suppose the flying pads can carry more fuel because the flyer doesn't have to bear the weight of it.
It could only fly for about thirty minutes, as I recall. Than it became dead. At the time, they were probably too expensive to leave begin when they ran out of power.