Halo 3 ODST was the first of the halo series I purchased as it was the first available on xbox 360, as the previous ones were for the original xbox. I bought it second hand on ebay as I wasn't sure whether I was going to enjoy it all that much, but I'm glad to say that I did.
If you're one of those people with common sense and haven't paid attention or money on the Call of Duty fad, then you'll appreciate this shooter game as it basically has the same concept of shoot and kill but personally I believe it has a better story line, better scenery and cooler weapons.
The game consists of two discs; one for campaign mode and one for on line multi player (providing you have a valid xbox live gold subscription) I wasn't particularly over joyed at the campaign mode, particularly in the beginning as I didn't appreciate the scenery and the dark views (despite turning the contrast right up) however it did provide me hours of game play that took a week or so to complete - and thankfully the scenery and game play improved as you are able to move around a number of areas fighting alien enemies that have tried to claim territories.
You have a number of guns to choose from and a small selection of melee weapons. My personal favourites were the energy sword; a sword made out of an electricity type compound that kills the majority of your enemies instantly (depending on their size and health) and generally looks awesome too, the gravity hammer which is extremely powerful and effective and can wipe out a number of enemies in close contact, The needler which is an enemy gun that you can take off them once you've killed them. It shoots out purple needle crystals and if you get an enemy stuck with a certain amount of needles they will consequently explode. There are also grenades and sticky grenades available on most levels which come in very handy.
On line multi--player also gives a number of different on line play to choose from; 'lone wolves' which is you playing against another team of individuals (basically every man for himself) or you can play as a team or doubles against another team of the same size. There is a handful of maps to choose from or alternatively once you develop the know how, you can create your own custom maps - all of which can be a private game with you and your friends or with strangers on line.
There is always someone who wants to ruin a game for you in some way or another so if you have a team mate who decides that it is a great idea to do some team killing (classed as betrayal) it will announce it on screen and if continued you are given the opportunity to kick this player from the game, although it doesn't help when you're on a roll in the game and someone ruins it - it's a relief in knowing it's not going to last long and you can wave them goodbye in a matter of seconds.
There are also features such as invisibility, where you can literally become invisible and sneak up on enemies, of course this is providing they didn't spot you going to collect the invisibility capsule, otherwise they may bombard the area with bullets and grenades and once you're hit, your invisibility isn't effective and they are then able to spot you - it's great to add a bit of sneakiness and suspense to the game with this. But be warned you may find yourself rage quitting if you're on either end of the spectrum, you may be invisible, but you have to be cunning too.
The game can be found second hand on eBay for around £5 now so you literally won't be disappointed, the servers have dramatically reduced in numbers since the release of halo reach but there are still hundreds of people on line that you can play with, so don't worry about the lack of players as there will always be a good bunch on line.