The art is what initially caught my eye on this book. Both the cover and interior artwork are extremely evocative. The creative commons licensing is another big plus -- I really admire the trust that they're willing to place in their fans. A third perk is that Posthuman incorporates any errata into their PDFs very quickly, so there's no cross-referencing multiple documents when you play.
But those are just icing, really. The most important elements are the system and the setting. The game uses the short fiction, Lack, to draw the reader in, and then goes on to detail a very nuanced post-singularity future in which transhumanity is recovering from near-extinction at the hands of their own creations. While I found myself most drawn to the socioeconomic conflict at the heart of the Martian terraforming effort, there's a little something for everyone in the setting, from the corporatist Planetary Consortium to the autonomists and anarchists of the outer system.
Character creation can be complex if you're not using one of the sample characters, so be prepared for that to take a little time (and ideally, track down an Excel character sheet to help keep track of things), but the system runs very quickly in actual play. The game is flexible enough to handle virtually any type of character, with normal characters including humans, AIs, and animals raised to sapience who may have an assortment of biological or synthetic bodies (or exist as an infomorph, with no physical body at all.)
An interesting and evocative setting, a system that runs quickly in play, and a company that's extremely fan-friendly? I'm hooked.