Well worth the money.
Honestly, it may be a tad overrated, but overall, the hype was worth it. (Although the campaign is based on opinon.)
If you enjoyed the other Halo's you'll enjoy this. If you have never played a Halo, you'll still enjoy it unless you've been living under a rock.
Multiplayer- It's just a beast. Most of the maps are balanced, and you can have fun fights over the power weapons. Not to mention the wepon variety is just fun to experiment with and come up with awesome strategies. The only downside is that you MUST learn to BR, or you might as well dig your own grave.
Campaign- I honestly though it was better than Halo 2's, but worse than Halo: CE. Although the campaign is fun, you can't help but realize that' there's little to no challenge in getting to the objective.
Other Features- They definitely add more value to the game. I mean there's no limit to what you can do in Forge and Theatre. I really find some of the games people make are really fun, and the hilarious pictures on file shares keep me entertained for a long time.
All in all, I think Halo 3 is a must buy for 360 owners. Don't believe me? Well then you don't have to because it's your lost.