WOW (no not world of warcraft)
Pros:
Great campaign with great storyline
Excellent multiplayer
Great enemy AI
Excellent soundtrack
Cons:
Sucky ally AI
Lots of noobs in multiplayer
No co-op
A bit too overhyped
Both the campaign and the storyline along with it is great. You will immerse yourself into it in no time and have fun shooting down the aliens. It has an excellent, competitive multiplayer as you can play with 15 other players online on the internet.
Also, the enemy artificial intelligence is great. The aliens called Elites try to dodge your attack, and when low in health, they try to hide or even charge you as a last-ditch method. Grunts, although annoying, usually are found running around when their Elite is dead.
Finally, the game has an excellent soundtrack which perfectly fits the situation in the campaign. Marty O'Donnell has done an excellent job orchestrating it and received many praise from the community.
There are some quirks about the game. Although the enemy AI is good, your ally's are not nearly as good. They do roll around to try to dodge grenades, but it is usually too late. They flinch heavily every time they get hit by plasma fire, and especially on legendary, they are only a little more than distraction.
There are a lot of noobs and people that will hamper your experience during multiplayer gameplay. Generally, they won't bother you too much, but once in a while there's one that is sure to annoy you.
Unlike the Xbox version, there is no co-op, but for me, I don't find it too be a feature that ruins the game.
All right, I will be one of those people that will admit it, but I think this game is a bit too overhyped. It is a great game, and a well-executed FPS, that's for sure. But people think they are like the "best game ever" and "the greatest FPS", when honestly, it's not. And I think that's a fact. There are other, more revolutionary FPSs, like Doom, Goldeneye, Quake, etc.
But I also think the things that Halo did are far more than what an ordinary game would do. Without Halo, the Xbox might as well drain in popularity and not be what it is right now. Halo popularized the Xbox and have turned it into a video game powerhouse that is today.
Halo: Combat Evolved is an excellent game, and I recommend anyone to try it, since it is cheap right now and it has good availability.