I'm going to get the hard part out of the way first and say that if you enjoyed playing Halo: Combat Evolved, Halo 2, or Halo 3, then you will also enjoy playing Halo 3: ODST. If that's what you wanted to know, there you go. I'm happy to be of service.
The game is easy to get into, the fun and excitement are present from minute one and the whole experience of playing it feels like coming home after a long, hard day to the smell of cookies baking and the sight of your favorite dog wagging his tail. It is, in two words "more Halo," and if that's enough to rock your world, then prepare to be rocked.
If you're still not convinced, then keep reading. I'm going to tell you more about why you'll probably love this game to death, but I've also got some thoughts to share about why, if you're like me at all, you might also find it disappointing. Because that's where I find myself with the game; somewhere in the middle-distance between love and hate, and teetering near the startling precipice of apathy. Let me put it this way: There is good sex and there is bad sex, but on the whole, sex is always, to some degree, awesome. That's how I feel about Halo 3: ODST.
Let's get the good stuff out of the way first: Halo 3:ODST, like all of the Halos before it, is the very definition of "competent shooter." The controls are phenomenal and every square inch of the game feels expertly balanced, finely crafted and designed to within an inch of its life. This is what Bungie does, and they do it better than anyone else.
The game is built on the Halo 3 engine and, like Halo 3, looks fantastic. This isn't a game about lavish scenery, but it may surprise you with some of its inventive and beautiful environments. The Dutch level, in particular, with its sand and sunshine, mixes it up just enough to help you forget you'll be looking at many of the same walls and floors for however long it takes you to complete the game.
As far as weapons and enemies, this is where it helps to have enjoyed Halo 3, because you're not getting much that's new. The notable exception is the "silenced" sub-machine gun, which is good for a few laughs, and does help you avoid detection if you're smart with it. Why you would bother with avoiding detection is kind of the whole shtick of ODST. In case you haven't heard: You're not playing as Master Chief. You're not even playing as anything similar to Master Chief, i.e. the Arbiter or one of the other few "Spartan" warriors. In Halo 3: ODST you're just a dude. Several dudes, actually. And perhaps one chick.
ODST stands for Orbital Drop Shock Troopers, which refers to the elite squad of soldiers who hurl themselves at the ground (or other ships) from orbit, aboard rocket pods. In ODST, you play as each member of a squad of ODSTs, and the story flips from perspective to perspective as "The Rookie" spends a desperate night trying to track down the other members of his squad who have been flung off course and landed deep in the heart of New Mombasa during the events depicted in Halo 2.