Good, but the new features are hardly flawless, and the campaign--while initially fun--is nothing to write home about.
The campaign in Halo 3 suffers more so than ever from this aspect. there are tons of new weapons and items to utilize, and this is fun, but the story itself--much like in Halo 2--doesn't deliver that "feeling" that the first Halo managed to capture. This is its biggest flaw, along with a lot of backtracking and an overall unbalanced fray. I recently played through the entire campaign co-op with a family member of mine, and I was completely bored the entire time. Directly afterward, I played through many levels of Halo 1 with him, and had a much better time. Despite the new nuances integrated into the newer Halo games, Halo 1 seems to be the one that really "clicked", despite its inferiorities when compared to the newer games on a mechanic or technical level.
The multiplayer in Halo 3 is where it earns its real points; it proves itself better than anything previous in the series, and this is a very good thing, considering the hype and popularity the first two games received regarding this aspect of the game. My only quibble concerning the multiplayer concerns the maps, which aren't up to the quality of the maps that shipped with Halo 2. Still, this isn't a big concern to me, as--at the time of this review--a new map pack has been released, with another one planned to be released very soon.
Halo 3 boasts 2 initial new features for the series: the Forge, and the Theater. While both are good ideas, many prove to be a bit more tedious or limiting than they should be--especially Theater.
With Forge, players can edit many aspects of the multiplayer maps, and can even hold matches simultaneously. This is fun for custom-game junkies, but otherwise isn't anything particularly special. Perhaps if Bungie incorporated it into matchmaking (social, of course), playing random Forged maps and things like that, it could be more interesting. Or perhaps a Forge mode in matchmaking. That'd be especially interesting...
The Theater mode is a neat aspect in that it allows a player to play back any footage of a campaign or multiplayer session and to watch the session from many angles. The reason it is flawed--and this mostly goes for campaign Theater--involves the fact that the controls are laid out in a menacingly annoying fashion, and there are extreme limits on what can be done in campaign as opposed to multiplayer Theater sessions (example: video selections can be taken in multiplayer, but not in campaign). This is an interesting choice, and something I'd consider a design flaw, along with the controlling aspect, which could've been done much more efficiently.
So, overall, Halo 3 is little more than a big package for awesome multiplayer. you have the good, but not great, campaign, which boasts some hidden things to find and unlock (which'll take the average player maybe another 2 or so hours...). You've got the newer features, which, if they had been integrated or designed more efficiently, would've been something stellar; however, as they are in the game, they are fun, but nothing extremely special. And you have your multiplayer package, which is great, and is rivaled only (on 360) by Frontline's COD4 (which actually surpasses it in my opinion...).
Still, Halo 3 is a solid package that likely does enough to please fans of the series (of which there are many), but doesn't necessarily achieve its full; potential, and this is rather as shame, as it is likely the final Halo (FPS) game...Ah well, it was a fun run. :)
And there's always multiplayer.