As a bit of extra world-building for players of Mass Effect, this book succeeds. It brings new dimensions and background information to the world you're basically thrown into in the first game, and it does well in fleshing out the backstory behind Captain Anderson and Saren. I don't think this book is very accessible to anyone who hasn't played the (wonderful and addicting) Mass Effect games, though. The game does a much better job of introducing the player to the world, and though the book has a lot of exposition intended to remind or introduce a reader to certain plot elements, it's just not a very good introduction. There's a bit too much jumping around, the descriptions of the various alien races can be confusing, and it's just much more fun to follow the characters along if you as Commander Shepard have already "been" to the places they go.
Of course, there's very little suspense here if you have played the game. If you've had your conversation with Captain Anderson about his bid to become a Spectre and his past with Saren, you know where the book is headed right from the start. But the introduction of characters Anderson doesn't mention, like Kahlee Sanders, keeps it interesting, and knowing a little bit about Saren's motivations (cliched though they may be) helps to flesh the first game out a bit. All in all, it's a light but worthy read if you are a fan of Mass Effect. Especially if you just finished Mass Effect 2 much more quickly than you expected and are left wanting to play more but have to wait until the third game comes out... but I'm just speaking from personal experience there.