It's entirely possible to complete ODST's campaign, even on Legendary, in a day if you have absolutely nothing else to do, or to phrase that in hours, easily under eight if you've even a modicum of skill. By itself that might not seem like good enough value and indeed, ODST was originally meant as an expansion pack, not a stand alone game. Even the name gives away the nature of the game - notice it's Halo 3: ODST, not Halo: ODST. Nonetheless it's an involving and entertaining enough experience to make the ride (and cost) worthwhile. But that's not a necessary justification because there's more to Halo 3: ODST than just the single player (and co-op) story.
There are in fact two discs in the ODST case. On the same DVD as the campaign is Halo's answer to Horde mode in Gears of War 2. The basic principle remains the same, fight off wave after wave of progressively tougher enemy forces. However, unlike Horde, Firefight has no endgame making your only objective the inevitable death of you and your teammates and the gathering as many points as possible.
Seven lives are shared between all members of your squad, of which there can be up to four, with extra lives unlocked every 2,000 points scored. As a further twist, the Covenant you face each round are determined at random so it's hard to be particularly prepared and various combinations of Skulls may be activated, at random, as you progress from round to round, in addition to the Tough Luck skull which is always on.
It's exactly the kind of fast-paced, hectic, and adrenaline-filled gameplay Halo excels at and I absolutely love it. If the success of Horde mode is anything to go by, I think it's safe to assume Firefight will be seeing a lot of play time in the coming months - if only from me!
The second DVD houses the entire multiplayer part of Halo 3, including all of the currently released map packs giving access to the an already thriving online community, and giving rubbish players like myself a much needed influx of new players of a similar skill level to pay against.
If you have, like I, already acquired all of the Halo 3 map packs then you might feel a little jaded at having to purchase them again. Honestly though, it's not that big a deal and if it means more players then I'm all for it. It's been a long time since Halo 3 came out, so you can hardly claim to have wasted your MS Points. Besides which, as I already hinted, there's plenty more to ODST to justify its cost.
Halo 3: ODST is, perhaps, a little flawed in places but fundamentally it delivers exactly what it promises: the same brilliant Halo gameplay we all know and love, but from a different perspective. You'd be mad to miss out on such excellence.