While this works best with Paizo's Pathfinder system, any Dungeons & Dragons 3.5 edition or d20-compatible game can benefit from this deck. The deck replaces normal critical damage with randomly drawn cards. Each card details four possible special criticals, one for each type of damage (slashing, bludgeoning, piercing, magic). These special criticals give combat more flavor and are slightly more powerful than standard critical hits on average.
The only thing to keep in mind is that Pathfinder Society games are not allowed to use these cards, but if you're running an Adventure Path, non-PFS-module or homebrew game, they're a GM option that I would highly recommend.
For that matter, if you're running a home PFS game, no one's going to complain if you include these cards, anyway. It's just public games that are likely to be a problem.