I do not usually review music, because it is an extremely subjective form of art. So, I am writing this as a fan of the original Halo soundtrack.
First of all, I give high praise for Martin O'Donnell and his associates, because that portion of the CD is fabulous and breathtaking. If you've listened to the first Halo, a few times at least, you'll most definitely notice that some of the music is essentially the same, but made more colorful by a wide assortment of new instruments. Especially impressive is the use of the guitar in at least two tracks, numbers 1 and 12. There are several completely original tracks as well, and two have been used to advertise the game widely to the public, so they will sound familiar. But out of all of them, track 2, Peril, is my personal favorite. It adds a sense of mystery and danger, while managing to sound almost cheerful in its presentation.
As far as the inspired music goes, I will not bash it thoughtlessly like some reviewers here. Again, it will be mostly subjective, but several of the tracks seem to fit into the genre, and sound almost like ending-credits types of songs, like Blow Me Away. Never Surrender sounds more like a techno/dance/trance mix and I don't know why something that makes you want to put up your arms and dance is in there. I happen to agree that they don't belong intermingled with the instrumental music. I had the fortune of being able to burn and rearrange it, putting all the rock at the end of the CD, and it definitely sounds a lot better.
Those minor gripes aside, I feel that this Volume 1 stays true to the core story of the game, and is definitely worth buying if you liked the first one. I do hope that Volume 2, the mystery of its release date looming over me like a dark shadow, includes everything else that was left out. Both of them together will make a complete and unforgettable soundtrack.