I finally jumped onto the huge lumbering hype bandwagon that is Halo 3 yesterday, when I picked up a preowned copy of the game along with Bioshock for £40 yesterday. After having played the game almost non-stop since I bought it, I am going to have to admit that most of the hype is justified (though I still think the best game of the year is Super Mario Galaxy.
The single player campaign manages to capture the essence of what was great about the original Halo, and fixes things that Halo 2 did wrong. Likewise, the addition of experience points to the multiplayer mode adds a whole new level of depth, and I love being able to watch real time videos of your last 25 games in the Theater. Forge mode sounds great too, but I haven't really messed around with it much yet.
It will be interesting to see whether I keep playing the game when I have to go back to work, and I reach my skill level plateau where I can't get any further. At any rate, the game will get regular use thanks to the fact that everyone I know who owns a 360 also owns a copy of the game.
Finally, here's a screenshot I took at the start of my first Team Slayer victory, when I'm about to start kicking arse!