Halo Zero is a fun game but it's just not the best.
Halo Zero is a prequel to Halo Combat Evolved based loosely on Fall of Reach. You play as Master Cheif once again and you fight the covenant of course. Probably the worst thing about the game is that it's not a 3-D first person shooter. It's 2-D side scroller. It turns out not to be a big deal after you find that the controlls are easy and you can use all of you're favorite weapons.
Halo Zero has a more or less cut-scene at the begining of a level that are really short and don't provide to much if only to turn the story in a slight bit. You can even do without them if you want. The game is really short though and may only take, difficulty or what you're skill in games is, about 3 hours at most.
All in all Halo Zero is a good game you'll just wish there was more to it.