I love the Halo series, and was eagerly anticipating the release of this. Was enjoying the first-person solo game of it, but like other reviewers, found it very repetitive after a while. I thought I may have been 25% through the game, when the credits rolled after one of the missions. My jaw hit the floor. It's SO short! Granted you get a multi-player disk with it too, but having already paid hard earned cash on downloading new maps for Halo 3, I've not found a massive difference in the new Multiplayer disk with ODST. Granted I'm still playing on-line with it, so we'll see if something brand new appears, but my hopes aren't great.
In summary, if you're a Halo 3 fan, save your money and borrow a copy of ODST from a mate and you'll be finished it in a few days.