Not overrated, but not a game I'm going to play for a year.
However, the multiplayer campaign play looks pretty cool, but I do not see myself playing this game again and again which is what you would have to do.
If you are only a single player fanatic, then it's a good game and good storyline. It's definitely not overrated like some may suggest, but not a game you are going to play and replay for a year. The single player gameplay is not difficult - I played through pretty quickly on Heroic. I had no desire to replay though, so I cannot say how difficult Legendary would be.
In that way, it is only an "8" for me. I assume the people that give it a "10" are really into the multiplayer or uber-Halo fans - either of which would get the replay enjoyment out of this game that I do not.