I've read the Fall of Reach and the Flood, and I'll be getting around to reading First Strike soon. What really struck me about the Halo novels was that, in most cases, books that come from video games are usually poor or average books. With the Halo novels, they are quality sci-fi books! You really don't even need to be a fan of the video games to enjoy them. They've certainly made me enjoy the games more. They add a whole new layer of depth to the story that I just love.
To anyone that hasn't read them : pick them up and give them a try. If you love Halo, and you enjoy reading, you'll love them!