The future is a big and scary place for some of us. For others it a place with no boundraries and as many opportunities to discover new things as there oppurtunities for these discoveries to kill us. Literally hundereds of futuristic novels, short stories, movies, television programs, radio programs and cartoons have been made and dedicated to the future or at least the futuristic. In fact the future is so big no role playing game can ever hope to encompass all of it. All it can do is give us the GMs and Players some tools to forge our own stories in the worlds of tomorrow.
Roleplaying is all about collective story telling. And some of the greatest stories are seen through the lens of science fiction and science fantasy. This book gives you more tools to forge those stories. You need D20 Future to get the most out of this book. Future Tech would be nice but is not required.
Whereas Future was really just a basic overview that was expanded in the subsequent books in varying degrees of detail, most of that was more geared for the GM who runs things behind the scenes than for players who actually play in these universes. Future Companion is geared towards the Player almost exclusively. More ways to get the most out of the mutation rules from the original future, more occupations to give an idea as to what the heck brought the character to beginning of the story and the obligatory gear and classes.
On a personal note, I have been buying D20 Modern books from Game Mechanics since they first put out Modern Players Companion and I have yet to get a bad book from them. Future Players Companion is no different. Many of the people that worked on the Future books help make this as perfect a primer for the game as their first book was. As a player, this book gives me a wealth of options for my character to be the best he or she might be in the collective story. As a Game Master this book is a wealth of information for my own NPCs and to help my players finalize their characters so the whole game runs smoother. It also helps as a spring board of new ideas, especially possibililities I would never have thought of otherwise. If you seriously want to D20 Future, your campaign will be all the poorer for not purchasing this book.