Halo 3: Orbital Drop Shock Trooper is a great game and it's worth playing, no doubt.
Campaign is very different from Halo 3, that is why I have been through such a great experience. While being an ODST you are much more vulnerable than a Spartan and there isn't any run in there and kill everyone you see, like there was in Halo 3. There are more tactics and silent attempts instead.
Being the ROOKIE you have to explore and find clues about your ODST's squads whereabouts because you have been given incorrect orders which have made you drop on Mombasa's (?) streets instead. While going back and forth through flashbacks you play as DUTCH, BUCK. MICKEY and ROMEO to retrace your steps and to finally find what the covenant have been up to.
With Firefight (Similar to Horde on Gears of War 2) being a fantastic mode to play with friends which makes you really enjoy the true ODST experience.
Your now wondering why I only gave this an 8.5, well I took some marks because of there wasn't any Firefight matchmaking which would have made it much more fun. And the fact that the campaign was quite short.
$68 (AUS) isn't a lot of money, but if your have bought it with that kinda of cash, that is money well spent!