Good but very, very over-rated.
"Combat Evolved" describes things very well. The combat in HALO is indeed a step up from most FPS' and is challenging and fun (until The Flood shows up, see below).Enemies will duck and dive for cover, engage you in a variety of ways and generally put up a good fight. What may even be more amazing is that your own team is not entirely useless. It may be one of the actually ground-breaking features of Halo that your own team actually put up a good fight and help you out by making themselves useful. No standing around looking curiously at the primed grenade at their feet or running into your line of fire (well at least not too much). They actually engage and kill the enemy effectively! On the other hand, maybe this say more about the incredibly sad state of Ai in gaming than it say about the AI in Halo.
The vehicles are also very well done. It's immensely fun driving around in the Warthog with a gunner while fighting aliens or laying down the pain from a Banshee fighter. The Warthog, fun as it is, has somewhat odd driving characteristics though; it skids like Bambi on ice even when you drive on dry concrete.
Halo also has my favourite type of cut-scene: the in-game engine cut-scene. Not only that, they're also reasonably well directed with a certain flair that gives them a movie-like quality. No talking heads here.
One positive thing that actually surprised me was Master Chief himself. I had expected a DOOM-style mute hero but it turns out master Chief talks and actually has a personality. Granted, he doesn't have the "charm" and "charisma" of Duke Nukem but he's no Gordon Freeman-style mute either. I really don't care much for the personality-free protagonist style of Half-life and DOOM. It works in DOOM because there's really no-one around for your character to talk to but when there are people talking and addressing you and your character just says nothing it just gets a bit weird.
The Covenant is an interesting collection of alien races with unique traits. I almost feel bad every time I shoot a grunt. With their runt, short appearance, high pitched voices and the clumsy way they carry their weapons, especially when panicking, they have a very child-like appearance to them. In some ways the grunts are more disturbing than The Flood. Unfortunately we get to know almost nothing about the Covenant and its alien races other than "they want to kill all humans".
It's also a plus that there are no boss-fights, just increasingly difficult battles.
The combat that was so fun against the Covenant goes right out the Window the moment The Flood rears its ugly head. What we have here is classic DOOM style enemies that has absolutely no combat awareness and who thinks that the old cavalry charge is the pinnacle of attack tactics. The only way by which the flood challenges the player is by sheer numbers.
While we're on the topic of The Flood , that old standard-issue parasitic alien life-form that has been the staple of Sci-Fi since the 50's, they're just not very menacing or scary. I have a hard time picturing it as such a monumental threat to sentient life that such extreme measures that we are presented with are necessary. I mean, the basic form, which looks like a mildly mutated jellyfish, can probably be killed with a peashooter. System Shock 2 did this much better and The Flood pales in comparison to The Many.
The Flood also brings to light some lazy writing and plot holes (spoiler warning).
How come The Flood is isolated on Halo? How did it survive for 100000(!) years without food? If the Forerunners managed to contain it why not just wipe out the last remaining colony instead of this monitor AI and containment protocol nonsense? Or did they keep them in some kind of storage for future use? Master Chief solves this problem elegantly by just destroying Halo but as we'll see in the sequels there are other colonies (collections?) of The Flood on other Halo-type installations. Really? Seriously, what the hell where they thinking? "Lets contain this extremely dangerous life-form in select installations across the galaxy and if the shit hits the fan lets just wipe out all OTHER life". Great plan guys, you really thought that one through.
When we are on the subject of the Forerunners we should not forget the impressive yet incredibly dull level architecture. Sure, the alien architecture is grand and immensely large (why so large? Where they 20 feet tall?) but it is also incredibly dull and boring. You'll be fighting through an endless line of dull, gray dimly lit rooms (that comes in very limited variety) all connected with the same type of zigzag corridor. While the symmetric design might make sense and be "realistic" it's also very, very boring. Quite a few times I just had to stop playing and go do something else for a while because I couldn't stand seeing yet another clone of room-design X. The interior room design also makes it very hard to keep track of where you are and where you are going. This is especially true in the part where you have to follow Guilty Spark. Most of the time I had no idea where that crazy AI was and I just followed the path where the monsters where since game-logic dictates that that is probably where I should be going. The Covenant levels are a bit better (they have colors!) but they suffer from even more and longer identical corridors that you have to fight your way through. Also, why does all aliens in all games design their ships and installations so that it is so very complicated to move from point A to point B, even when A and B are physically close? Halo suffers from this although at least it's nowhere near as bad as the stupidity that is Xen in Half-Life. They do make solid stuff though, the aliens. These installations has stood without maintenance for 100000 years and most of it works without a hitch. Amazing.
The Covenant is also a bit anonymous. Who are they? How did their religious worship of the Forerunners start? What are the specific of the different races? All we get is a few sentences in the manual and that's it and the only way you'll ever interact with them is in combat. The Kilrathi they aren't.
One final thing. Why can't Master Chief run?
The Bottom Line
If you take it for what it is, a very well-made FPS with a decent but also engaging story, you won't be disappointed. While not as revolutionary and ground-breaking as some might want you to think it's still a good game and if you can muster up the courage to stand the dull and repeated architecture you'll probably enjoy it.