Pure appreciation for the classics and their influence on modern gaming are what make these types of games fun for me.
With that said, however, I've always been the type that enjoys learning about how some of our modern favorites came to be. And being an avid Halo fanatic I found this to be an interesting experience. And giving that this game is over ten years old I'd have to say I'm suprised; it was actually entertaining. Don't get me wrong. By todays standards this game has little replay value and the novelty will wear off soon. But for a couple bucks this game wasn't that bad of a deal. The game is at its best when you're just mowing down waves of aliens with the Assault Rifle. However, you always feel compelled to save your ammo for "bigger battles" that never come because there really isn't enough ammo to spray and pray your way though it. And all entertainment just bottoms out when you run out of ammo because the melees are pretty much worthless. Of course there is also the level design to talk about. The game is more of a maze than an all out FPS. You'll often find yourself running around like a chicken with its head off. It's actually kinda funney how a game could have you running around in a circle and still trick you into thinking you're going somewhere.
All in all, most will probably find this to be a novelty. Getting it for the sake of seeing that there was a BUNGIE before Halo. I'd have to say the score I'd give it is about in line with most XBL Arcade games. They're fun for maybe 30 minutes at a time but you just can't play them all day long. And for someone who's looking for a break from the usual and wants to go old school for a minute, Marathon just may be for you