Everything's the same, yet everything's different
What is different is...well...everything else. You're no longer a super-human spartan, but an Orbital Shock Drop Trooper that is "merely" the best of the best of what mankind has to offer. But is that enough to impress the Covenant?
The story is more or less set right after the opening act of Halo 2. The city of New Mombasa is in ruins and controlled by the covenant. You are inserted from orbit (hence the term ORBITAL drop shock trooper) when intense atmospheric disturbances throw both you and your squadmates off course. After a rather improvised landing (read: crash) you regain consciousness about six hours after the drop. Battered, bruised...and alone in enemy territory.
The gameplay will feel familiar to anyone who's played Halo 3. The controls are similar and so is most of the interface. The most significant difference is the map which you access by hitting the start-button. The map is invaluable as it not only helps you traverse the urban jungle, but also keeps track of enemies in real-time. This is useful as you WILL bite the dust pretty fast if you try to tackle this like a spartan. Instead, you will have to plan you're approach to enemy parties. A well-placed grenade or round from a sniper rifle still won't make taking down three or four brutes easy, but it'll sure improve the odds.
The profound solitude that you feel skulking through an occupied city is interrupted by flashbacks that recount the actions of your squad while you were unconscious. These typically revolve around either a set-piece or some form of action sequence. Either way - they provide a great source of variety to the gameplay.
I personally found the campaign immensly enjoyable and a breath of fresh air. Set against the bombastic exploits of Master Chief, the experience of one soldier (nicknamed "the rookie") provides a stark contrast. The plot is not nearly as convoluted as the interplay between the forerunners, covenant and flood, but there are a few twists here and there. It works and is beautifully executed with the help of some grade A voice talent.
I'd recommend ODST to just about anyone, but obviously Halo-veterans will get the most out of it. Lock n' load, soldier. It's time to show the Covenant that "the demon" isn't the only human they should be worried about!