It's a fun game,it has some of the elements that made the first Halo game so great.
The enemy AI in Halo 3 is very defensive and the enemies like to hold their ground and make you grind through areas(without carelessly rushing at you like in other FPS games).There's some good new powerful enemies such as enemies who'll try to pound you with giant hammers and they will constantly chase you and being struck by their giant hammer can easily result in instant death.There's some giant 4 legged walker things(similar to the ones on Star Wars)and they'll shoot at you of course and the only way to destroy them it to climb onto them and destroy their reactor type of thing and you'll need to get far away from them before they explode.Some enemies are well hidden amongst natural surroundings(even snipers)and this can lead to some exciting and long shooting/sniping duels.
However,besides the very short campaign and on rails aircraft segments,the other flaws the game has IMO include,annoying segments in levels such as when you have to battle tough enemies but no extra ammo is available in the area and you can't leave the area because you arrived there by an on rails aircraft segment,and because melee attacks are useless against those enemies,you'll have no choice but to reload from a previous checkpoint.Also some enemies that crawl on ceilings can only be killed with long ranged guns and it's easy to run out of ammo for those guns and those enemies can easily kill you if you don't kill them quickly and I found this frustrating.Also,the AI controlled allies drive the vehicles that you ride in extremely poorly and will drive straight off cliffs,causing you to die and make you redo long sections.
But nonetheless,the game is fun for the most part.
The graphics were nice looking for their time and for it's time it was easily the best looking Halo game(technical wise)because of the beautiful realistic detail in the vegetation,soil,flowing water,landscape structure and the desert environment had some nice looking sand dunes and there's some organic structures that you'll need to enter and they will move as if they're breathing and they'll even talk to you and it's kind of creepy.
The music keeps the beautiful and orchestric sounding Halo theme music and add some more beautiful music to it.
I don't know what else to say about this game,except it's fun at times but it's single player campaign is very short and is a bit disappointing considering the hype the game was given but if you like Halo's multiplayer,you'll probably love this game.