Heavily influenced by the old AD&D Spelljammer campaign setting, Starjammer provides a ton of great new options to take your Pathfinder game into space, or even just providing new and exotic character options in a more "grounded" campaign.
Let's just get this part out of the way first. As other reviews have noted, there are a few minor editing errors. These are not a huge problem with a book originally released as a PDF, but it is worth noting that you will likely need to update your copy with a revised version in the near future. Additionally, I found that the book would have been better served with another few pages on personal equipment.
As previously stated, there are a ton of new character options that can be used for a spacefaring campaign and that many of those options can apply to a campaign not set travelling the stars. This is the book's greatest strength. The material fits seamlessly into any existing Pathfinder rules. The next best thing is the very interesting fusion of technology and magic. Cloaking devices, for example, are built from magical purple crystals set in electronics. This creative theme is what truly separates Starjammer apart from the galleons sailing the phlogiston of Spelljammer. It is a more modern, but still very strongly fantasy theme.
Overall Starjammer is just shy of perfect. The flexibility and new options it provides allow for a variety of campaign types and the imagery of the tech/magic fusion prompts creativity.