This is a rollercoaster of value, depending on what part of the crowd you are in.
Halo 3 was a wonderful multiplayer experience, only matched by Modern Warfare at this point of the console generation. The multiplayer goodness is obviously here. Halo 3's Mythic disc offers the full Halo 3 multiplayer experience. All the goodies from Forge to Theater are still intact. If you never had Halo 3, buying this would be awesome, right?
But...if you are interested in an expansion/prequel to Halo 3, chances are you DID have Halo 3, right? Therein lies the strange problem in ODST. The campaign is perhaps the shortest Halo campaign yet. Experienced players or anybody playing in co-op will not take more than 4-6 hours to finish this. If you already know what you are doing, it can be cleaned up in a mere two hours. I personally completed campaign with a friend in less than three hours, and that was while collecting all the hidden audio logs. So you essentially have a campaign that can be completed in a sitting, or a really long lunch break. What else does ODST offer?
That would be Firefight, the latest new co-op craze. Like Horde, Nazi Zombies, Survival Mode, or whatever you want to call it, Firefight has players teamed together taking on wave after wave of increasingly difficult enemies. I will give Firefight this, it may be the best co-op survival mode yet. It isn't blandly uninspired like Nazi Zombies, frustratingly impossible like Left 4 Dead Survival, or ridiculously easy like Horde mode. It is just the right level of difficulty and requires just the right amount of teamwork to survive. The best part, you can't beat Firefight mode! The waves just go on and on, and it is essentially about setting records and just having fun. With multiplayer medals and a fun scoring system, this formula works.
However, it just isn't enough to warrant ODST being worth it. If you are a Halo fan, then this is a good game, but it's difficult to justify the purchase. If you never bought Halo 3 and for some reason you would like to get into it even though it's life span is on it's dying legs with Reach approaching in a year, then you will probably find ODST to be an awesome package. I can't really say I recommend paying $60 for this when you could instead pay $40 for the real Halo 3. Unless you want in on the Halo: Reach Beta (which comes on the ODST Campaign disc and will be very exclusive in 2010), there isn't much reason to have it. I wish there was much more I could speak on in this review, but it is literally two modes and then 90% of what Halo 3 already brought to the table. It's good, but for full price? It's damn near robbery.