A short, but enjoyable, campaign and Firefight mode make for a nice package, but not really a full one.
The campaign in the game is shorter than any other Halo, but there are no flood levels to despise. It took between 5 and 6 hours for me to complete a playthrough on heroic. While the story isn't exactly great, and ends rather abruptly, the gameplay is still superb. It feels a little more like the original Halo than Halo 3, and that works well. The subtle changes make the game feel unique, but still having that core Halo shooter experience that so many fans have come to love.
Firefight mode is the other addition. You fight wave after wave of Covenant troops, with limited ammo and respawns, in an effort to get a high score. Its a good challenge. There are 10 maps to play on which makes for more depth than expected. Still, this mode has a limited appeal. If you don't have some friends to play it with, then you'll probably get bored of it pretty quickly.
This game also includes the original Halo 3 multiplayer mode on a separate disc. All the downloadable maps are included along with three new maps. Unless you don't already own Halo 3 then this isn't all that exciting, especially since you have to switch discs anyway. If you are new to Halo, this is a well balanced, in depth, highly refined multiplayer experience which will give you many hours of entertainment. I expect most people buying this though have played Halo 3 extensively.
So, if you're wanting more Halo then ODST will hit the spot right. Its not as full of an experience, but what is there is excellent and unique in the franchise. Firefight mode is exhilarating and well made, but will not appeal to everyone. Then there is the Halo 3 multiplayer which most people already have an opinion on, if not own it outright on another disc. If this were at a lower price point it would be an easy recommendation. However, the recycled online, the length of the campaign, and the limited appeal of firefight will prevent this from being a value for everyone.