Instax is based on the Kodak Instant Technology. Compared with Polaroid 600 film, Instax is sharper, slightly faster (ISO 800), has more saturated color (I think), and keep better in storage (I have some from 2004 that still looks great. Polaroid films start to develop orangey well before that).
The downside is that because of its Kodak lineage, it is exposed from the rear rather than the front. This makes it impossible to use in an SX-70 camera, even if it was cut to shape. The other downside is that there are no 'nice' cameras which can shoot Instax. The only cameras available are P&S fixed focus.
It seems like it might be possible to convert a Polaroid roll film camera to use Instax. It would take some machining skill to make it look nice.