Played through the Peace Walker Demo, I finally figured out where to go. Anyways, the game is amazing so far, you can choose from 4 different types of body armor to wear depending on your playing style, it's 4 player co op, and very fun.
Yeah, when my friend got it, he received about 5 angry messages asking why he bought it. I don't see how it could not be more worth it. Maybe it's because they have to think a bit more in ODST then in Halo 3? XD
It's totally worth it, it has all the map packs, plus three new ones. The campaign is awesome, the characters are cool, and interesting, because we have been following (mostly) the same characters since Halo 1. Firefight is good fun, hell, its totally worth it in my opinion.