I am on record saying (again and again) that I don't like big gaming books. This is a BIG gaming book and yet I still want to run the game.
The things that make the game big make sense. For people who are struggling with the choice paralysis of the Fate Core rules relief can be found in all of these rules and examples. Sarah Newton has done the heavy lifting and created a worked example of how you use the Fate Core rules to do a science fiction game. For people who creating material from scratch is their strong suit, then Fate Core should be their starting point. If you are struggling with the hows and whys of the Fate Core rules, then Mindjammer may be the better choice.
All of the little things that you would need for running a science fiction campaign are in the book. Space craft. Aliens. Weird worlds. Alien cultures. Using Mindjammer as your starting point can really help to introduce the Fate rules to an audience other than people who are already fans of Fate. This is something that I don't think that other third party publishers have taken into consideration when crafting their games. Writing a game using the Fate rules for people who already understand those rules is easy, writing them for an audience not used to the concepts and ideas means reaching out to a wider audience, who can then be directed back to the core materials and other publishers. In Mindjammer, Newton manages to do this in a way that will neither alienate the existing Fate audience or confuse those new to the game.
The book is complex in places, but a lot of that comes from the science and setting that has to be introduced and explained to the gamers. Much of this complexity can be considered to be training wheels that can be kicked away as the group becomes more experienced and confident in their master of the rules. This is demonstrative of a canny design sensibility on the part of Newton.
Mindjammer pushes the bar up high for third party Fate publishers. Even before finishing reading the book, it is becoming my favorite implementation of the Fate Core rules.
If you're curious, the Mindjammer PDF is available here.