Halo 3: ODST, buy it if you're a true Halo fan!
Halo ODST's campaign is not bad, it's a bit short though, and just the same routine, and the cutscenes aren't interesting, they're quite boring actually, it's also very hard now since you're just a normal soldier rather than being Master Chief. So it's very easy to die and you should be careful about your stamina bar and you should always find med packs, they help prevent you from dying.
Gameplay
The Gameplay is better, the weapons have improved a lot, such as the sub machine gun, it's so amazing, aswell as the Pistol, it has aiming support now. The graphics are also quite good, but it's not the graphics that matters to me, it gets tough at times since you're not a Spartan, but you'll get used to it, its really awesome the gameplay.
FireFight Mode = AMAZING
I just tried out fire fight mode and I was thinking to myself, this game can't get any better, It's always action, non stop, if you've played Gears of War's Horde Mode, you should try Fire Fight, much more entertaining, huge maps and two players offline, its really fun like a lot of Brutes come and attack you and you and your team mate strike back and there's no stop in fighting, this is what made me love ODST the most, it's just super awesome!
Get this game if you think it looks amazing because it is, a lot of improvements hav been made, a lot better things to play in, 3 new maps, and all the same maps as in Halo 3, especially firefight, I guarantee you firefight is worth the while, if you play firefight, play in Lost Platoon, best map ever, well good luck and if you read this and get the game, make your own review.
To top it all, Halo 3 ODST is just pure awesomeness :P XD