I picked up the Halo Graphic Novel yesterday afternoon. It's got 4 stories in it and I've read the first. Each one has a different artist and writer, and all the stories are supposed to fit really well within the Halo universe, and really give alot of insight.
The first story, the one I've read, is about the Covenant Elite known as the SpecOps Commander (He was the elite in white armour that assisted you as the Arbiter in Halo 2) and his first encounter with the Flood. The art is very good, and the writing is top notch.
The second story, which I've read the first page of, is about the development and testing of the Mjolnir Mark IV armour. It's a very different art style that the first story, but it equally as well done and the writing seems to be just as good.
I haven't read any farther than that yet, but so far I can say that it's going to be a very well done book. In the forward it's said that they didn't want to make comic that would just ride on Halo's wake or just be random stories and characters that didn't matter. They wanted to make something cool. So for I'd say they did a damn good job.