Why would we need this?!
It makes you more vulnerable than in the previous Halo game which made it a lot more harder. In Halo 3 I was able to finish the game the first time playing on Hard, ODST on the other hand gave me a run for my money. You regain your health the same way you do in Half-Life, you find a machine and use it to patch up. Quite a good solution, I'd say.
The premise of experiencing the Halo universe through the eyes of a weaker protagonist sounded very interesting to me. Not just because they are weaker (I already mentioned that), but also because they are more desperate than Master Chief who could survive a nuke without ever showing some emotion.
Even though I never actually watched Firefly, I was quite amazed by the great voice-actors who deliver their lines brilliantly. If this is going to be the standard of video game voice acting than we have a bright future ahead of us. They also interact with each other in a believable way which in turn also made for more interesting characters.
This is the shortest game I ever played, especially when you keep in mind how long it took for this game to launch. I played through the entire campaign on Hard in the same afternoon the game arrived. The multiplayer did keep me busy for another two days, but that doesn't count for reasons I will explain later.
The sequences where you play as Rookie which act like a HUB-world of sorts are annoying as hell. There is barely anything to fight, it is really dark and compared to the actual missions they aren't fun either. Maybe if there was more light and they'd just tell me what to pick, I would have been kinder towards these sequences, but no, somebody on the team must have wanted it to be this way quite badly.
Firefight is an idea that has already been done (better) by Gears of War 2. The fact that all the enemies are delivered to you by a phantom which fires at you as well and is indestructible is also annoying, but what really takes the cake is the lack of fun or reason to play it. It isn't challenging and it isn't rewarding in any way. All you get is an achievement if you get 200.000 points, which might have been okay if that didn't take incredibly long. I could get these achievements without much effort, but I would just commit suicide in the game because I'd get bored.
People who already played Halo 3 are going to notice that the new multiplayer isn't new and certainly not worth it. It has all the maps that were already in Halo 3 (+ DLC) and comes with three new maps. Two of these new maps are rather small and the last one is normal-sized and fun in Forge mode. These are certainly not worth buying a whole game for because Foundry is still the better Forge Map and the DLC maps we already had were much bigger.
The Bottom Line
If you want to have Halo 3 and plan to play it online a lot than buying this game might save you a few dollars on DLC. If you already owned Halo 3 and you'd like to see the next game in the franchise... maybe Reach is something for you, I am not sure, didn't play it and not planning to. And of course this is a sequel in which the player partakes in events everybody already knows the outcome of.
Even if we don't compare it to the previous Halo it disappoints. The game only lasts a few (good) hours and is full of boring sequences that take longer than the mission they are leading you to. The voice-actors do a pretty good job and the gameplay is acceptable, but this is an expansion pack at best and a big DLC at worst. Not worth the money you'd pay for a stand-alone game.