If you are a fan of the Halo universe, it's almost impossible not to like this game...
In the campaign you play as a marine (who looks quite a bit like the Master Chief) who has found himself on a mysterious alien planet, guided only by a witty AI named Durandal. These singleplayer missions involve a lot of exploration in order to find new guns, and to progress through levels. The gunfights are usually massive, intense and extremely fast-paced. My one problem with the singleplayer is that it might be a little too non-linear, in that it's very easy to simply not know what the hell to do on a given level.
The multiplayer has the same sort of gameplay as the singleplayer, minus the exploration, so it's basically, fast-paced, intense, and massive gunfights. The multiplayer has a good amount of game types and match customization options (all similar to the Halo games), but I have one problem with it, which is the weird scoring system. Basically, if you die, you get -1 point and if you kill someone, you get +1 point, so there's no kills-deaths ratio here, which I didn't like very much, but it's not a very big deal.
Another cool thing about either the single or multiplayer is that there are so many weapons and mechanics that work exactly like Halo, which anyone who's played the Halo games will greatly appreciate. These similarities include dual wielding, similar weapons, and enviornments and characters that look almost the exact same as those in the Halo universe.
The graphics in this game are pretty good, and the touch-ups done by Freeverse are awesome. Textures are now better and the game runs and a smooth 60 FPS, not to mention that in the first place, this game's textures were amazing.
The sound in this game isn't too good, everything still sounds low-quality and muffled, which I think could be possible to fix, but still hasn't. But, the sound is still fully bearable.
The length of the singleplayer in this game is about as long as that of one of the Halo games, and the multiplayer will be pretty awesome for a few months, but I'm afraid, as with all other XBLA games, it's popularity will die down.
Overall, if you liked the Halo games, than it's very hard not to like Marathon Durandal, as it's been called many times "a mini Halo." The game probably has the best value of ANY XBLA game because of its popularity and the length of the campaign. Not to mention the graphics are great, and the sound is still passable. If you're interested in seeing where Halo began, look nowhere else.