Overrated- Yes...Great Game- Very yes.
First of all, Master Chief is back after his efforts still didn't stop the alien forces, and he is better than ever. There is also a nice little twist. For part of the game you play as the Arbitor, a covenant guy, but he is doing pretty much what you are doing. It's a little confusing. I never played Halo: CE so I'm kinda in a fog. It didn't help that I had the sound too low. Or maybe I just didn't want to watch the horrific cutscenes. Ew...
Anywhoo, the single player is excellent, and is still pretty chalenging even on normal difficulty, but not too hard that it's impossible. There are some new weapons and vehicles, but for the most parts, it's a lot like the original Halo, which is a good thing. This portion of the game will keep you waiting to see what's next as there are different types of levels and ways to beat these levels waiting around the corner. It took me awhile to beat it since I kept turning of my Xbox without saving on the flood level. Good thing that level was fun and challenging.
Halo 2 features some robust multiplayer options, icluding multiplayer on split-screen, system link, and XBOX Live. There are a ton of different ways to play each map, and you can tweak these options to fit your liking. Some of the multiplayer modes are slayer (Normal deathmatch), team slayer, capture the flag, and king of the hill to name a few. You can also customize your player and even make him a covenant soldier (in slayer).
The graphics were probably the best ever on a console when it came out last November, but now has been outdone, and the sound is very good, especially the weapon noises. The fact that you can replay certain levels in single player add to Halo 2's value, as well as the co-op mode and the great multiplayer mode make for a long time of fun. Plus it's always fun to split screen with a friend (rockets and swords come to mind).
Overall, Halo 2 is a must-play for anyone who has an XBOX. If you enjoy FPS's or if you liked the original Halo, Halo 2 is perfect for you. [UPDATE] Man this looks great in 720p widescreen running on my Xbox 360. Splitscreen is more fun too since you have more vision.