Sure, it's basically the same as Halo 3, but it lacks the magic and memorability that made Halo 3 just so great.
Simply, the new single-player campaign that ODST brings to the plate lacks the magic of Halo 3. It isn't nearly as memorable, nor as fun. The main difference between this game and Halo 3 is that you don't play as Master Chief. That's about it. The first-person interface is different, and the cast of characters is larger. You play as several different characters behind enemy lines in the game, with a regular return to the main character, The Rookie, who is speechless throughout the game. It's difficult to call him a character because I can't think of a single personality trait he exhibited, especially due to the lack of dialogue.
And since much of the credit for the fun this package offers can't be given to the new ODST itself, it can't really be scored particularly high. A fun new addition, however, is the Firefight mode. You, alone or in a team, must fight off many waves of incoming enemies and survive as long as possible. It's plenty of good co-operative fun, and does require strategy and planning if you plan on getting very far.
It is a fun game at heart, but it inevitably lies in the shadow of its predecessor in almost every way.