Ok, I have a few more things to say about this technique. First, while it definitely WILL have it's uses, I don't think it's going to be as useful as we first thought. At least not with everyone. The reason is this:
The ISJ can be done without executing any aerial whatsover. If you'd like to see for yourself, pick ness, and continuously jump, fastfall, and as you land hold up on the control stick. This will buffer a tap jump, and ness will automatically use his second jump from the ground. It's easy to see if you're doing it right because his second jump has huge gold rings while his first jump is just a simple hop.
That established, let's consider the pros and cons of such a mechanic. It just happens to be good with Ness actually, because if you do the ISJ and immediately execute a bair, ness will do a nearly grounded bair, followed by a huge rise. the gold rings will not appear, however his actual movement mirrors that of his second jump. It'd be great if you could do up a video of this magnificentmarth, because it looks really cool.
However, this has the disadvantage of taking away your second jump right from the get-go. So naturally, in some cases, it would be better to actually land, and do you attack from your first jump, preserving your second jump.
It seems to have different properties than the infinite jumps of boswer, luigi and charizard. While these characters obviously stay airborne while performing the infinite jump, they have to execute an attack in order for it to work. For instance, pick bowser, and do his ridiculously easy infinite claw jump. Now, try to jump, fastfall and hit up on the control stick. Bowser just lands like the big fat turtle he is. I may be completely wrong, but i think the ISJ uses a different game mechanic than luigi and bowser's infinite jump.
This just in: The ISJ is officially AMAZING with ness (i havent tested Lucas) Ness performs aerials with the momentum from his second jump right off the ground. I talked before about his bair, but now I want to mention his dair. if you ISJ a dair into say, Capt Falcon, ness will smash him right from the ground, the Capt will pop upwards, and ness will be carried by his second jump momentum right to the level of Captain Falcon, or somewhere in his immediate vicinity depending on your opponents %age. There are many more weird quirks to doing this, so experiment, and report. His fair has some nice properties as well.