Sometimes good, sometimes a disappointment
I have mixed views about Halo 3 to be honest. At times its been quite good, but at other times, I've disliked it a lot.
First of all, Campaign mode continues right from the cliffhanger Halo 2 left us in. Master Chief lands near New Mombassa, and has to continue fighting the covenant. To avoid spoilers, I won't discuss the rest of the storyline but I will say this. Its quite disappointing compared to Halo 1 and 2.
Halo 1 and 2 had a very good introduction level. The first Halo had a pretty memorable level where you escape the Pillar of Autumn as its attacked, and crash land on Halo. Halo 2 had a decent tutorial, which leads to a Covenant attack on the space station, and doing a very risky jump out with a bomb to destroy a spaceship. Halo 3 however, starts off with a quick helmet calibration, and jumps you straight into the fight. The first level didn't feel good enough though.
At times, you will also get random scenes, where the Master Chief will see a vision of Cortana or Gravemind in his helmet and will move slowly during the scene. Most of the time this is a good idea, but at times, this can get very annoying.
The graphics don't feel upgraded enough to me. It looks like a just noticeably more detailed version of Halo 2 on the xbox. The lighting is better though, and at times can look quite good.
The gameplay itself is slightly changed from Halo 2. Instead of holding 4 Plasma and Frag Grenades, you can only hold 2 of each type, which to me feels limiting. However, there are two more types of grenades, which until later in the campaign, you won't notice much. I also didn't like the choices of weapons as much as I did in the first 2 games. The Spiker feels ok but the Mauler feels disappointing. However, the Gravity Hammer is quite good.
The difficulty of the game, especially in coop feels slightly disappointing. In Legendary Mode, you'll have a high chance of dying solo unless you're very good at the game, but remember that you can have upto 3 other players fighting with you online.
Which leads me to my next point, Online Multiplayer.
To me, Online Multiplayer feels like its lacking in playlists. Sure you can have Team Slayer, Team Doubles, Lone Wolves, Ranked Big Team, Skirmish, and unranked versions of said playlists, but I still feel like its lackluster.
Playlists such as Rocket Race and Multi Team are only unranked, although I'd like to see ranked versions of them. Multi Team has also changed a bit since Halo 2, rather than being 3v3v3v3, its now 2v2v2v2, which feels a bit disappointing.
The community is also at times bad. The amount of times that I've joined a game, someone is shouting "veto" through their mic, and suddenly almost everyone vetos the map, just to have a worse map and game type take its place, is a bit annoying.
There is also the fact that leaving a game gives you an experience penalty. But when everyone else in your game leaves for various reasons (e.g. bad choice of map, disconnect etc), you'll have little choice but to either leave yourself, or get steamrolled and further risk getting ranked down in a ranked playlist.
Weapon balance is also at times an issue. When a whole clip of ammo from an SMG can't finish off an opponent, but they can two hit kill with melee, thats definitely an issue. The Energy Sword is also another example of an overly powerful weapon, where you can one hit kill someone who you're aiming at.
Despite these problems, i'd still say Halo 3 is fun to play.