ODST is a great new face of Halo, and Bungie. Totally worth while.
The game itself, while based on the events leading up to the conclusion of Halo 3, is completely different. You are a new character, you have a different objective, and more importantly you are not the super soldier Master Chief. The Rookie is a brand new face (and more importantly a realistic one) to the Halo world. He is not super strong, super fast, super endurant, and so the list goes on and on. This allows for a completely different style of gameplay.
The gameplay itself is designed to be far more stealthy. I noticed this greatly within the first hour of play. Unlike all the other Halo games where Master Chief can simply run in guns blazing and kill everyone, the Rookie and his squad-mates are in a completely different situation. The enviroment has to be used to a far greater extent than that of the original Halo 3. I noticed this by paying attention to minor details such as explosive barrels being next to mounted turrets, and gradually flanking the occasional Brute. This more discreet style of gameplay also allows for different weapons to become more useful, such as, the suppressed smg and pistol.
Another thing that became increasingly noticable during my gameplay was the A.I. of the Covenant. No longer do they concern themselves with idea that the Master Chief is a "demon." They will come in and search for you without fear. They also seem to react better in ways they did not in Halo 3. The Grunts in particular have shown a better knowledge of throws, and rather than it being one individual doing throwing two grenades, there tend to be more Grunts throwing individual grenades at the same time. The grenades themselves have become far more important than in the original Halo 3. Master Chief having a strong shield, and the ability to take a gives him the ability to take more charge. The sticky grenades have come in handy so far because killing anything bigger than a Grunt requires them.
Firefight mode is a blast, and while I have not been able to play with a group playing by myself was challenging. I am glad to see that Bungie adopted a unique style of a "survivor" form of gameplay. This gives it a whole new flair that until recently only games like Left 4 Dead had.
I was disappointed with certain things in the game that were very minimal. I was displeased with the fact that while you have supressed weaponry that the Covenant can find you so easily. I felt that this took away a great advantage that the ODST game was supposed to be about. After all, if you pull off a headshot on a Grunt without anyone seeing you then you should obviously maintain that same element of surprise. Another surprising thing, which was a disappointment, was the lack of other unique ODST-esque weapons. The supressed SMG and pistol were great ideas, but it would have also been very nice to have and something similar to a supressed Battle Rifle, or a smaller (and less damaging) form of a supressed Sniper Rifle.
All-in-all I give the newest edition to the Halo family a strong 9.5 due to the advanced A.I., the new weapons, the intriguing new style of gameplay for bothe the campaign and firefight, and most importantly the new face of the Rookie. This new game, and new character, offer an amazing amount of possibility for Bungie in the future. I hope that there will be more games like ODST.