"The Flux" expresses a simple but elegant idea: player characters live multiple lives, in multiple worlds, under multiple RPG systems. As someone who owns far too many games I've never played, it's a brilliant narrative device to justify putting away D&D/Vampire/whatever for a while and trying something different.
Past worlds disappear and others take their place, and the "soul" of a character (namely the player) reincarnates into a similar character under the new rules. Those old character sheets, however, still serve a purpose in a brave new world. Old worlds may return, and old lives pick up where they left off ... or not. More intriguingly, player character has a chance to remember an old life well enough to borrow its abilities, but in doing so risks the wrath of the current world.
Unfortunately, the GM may risk the wrath of his players, if (as the author suggests) he surprises them with a brand new world and brand new rules. As cool as the Flux is (as a concept), GMs need to know their players well enough, or need to have cowed them enough, that they'll go along.
For 14 pages of content $5 is a tad overpriced. The contents, however, do get the old brain-meats working.
[4 of 5 Stars!]