First of all, this is a very good game with excellent core gameplay. The shooting is great. There is a good selection of weapons and vehicles. The online customization options and modes will leave you spoiled for choice and entertained at least until the next legendary shooter comes out.
Now the bit for fans of Halos so far...
The graphics have changed for a start. People say they are better. I beg to differ. Everything seems more pixelated. I adored the smooth and artistic textures and lighting of reach. Everything here seems to have been sharpened up in some places and yet rendering is ignored in others. If you are familiar with COD graphic style they seem to be moving in that direction.
Everything seems lighter. Guns don't carry as much weight. The way the chief moves, navigates and fights makes him feel more flyweight fighter than bulky heavy-weight fighter. Explosions don't sound nor look half as good as they used to. They also give off very little impact/blast and do not affect the environment (smashing around crates etc) at all.
You and enemies take less damage to kill than ever, this carries over into multiplayer, but this of course can be modified thanks to the games massively impressive on-line customization options. I wouldn't be surprised if a lot of games stating they have halo classic settings crop up all over the place soon.
So far (over half way through) there haven't been any moments in the campaign that made me go "WHOAH" or anything like that, which I must say is disappointing, as other halos have impressed in that department.
Anyway I just wanted to warn hardcore fans of those changes. Ultimately this is still a great shooter with loads of content but it does feel like some of the feel of Halo mechanics have gone out of the game. Is it worth buying? Unless you are a big fan of the series I wouldn't. Get Reach. It's cheaper and I personally think it differs from other shooters more than this game,which can keep your collection more varied and thus you get bored of it less easily. It also has firefight (survival) mode which is customizable and this instalment does not include it.