People who have commented so far are douchebags, they arn't particularly helpfull are they? haha. Also that Matthew guy who previously commented is wrong... ODST can be considered a full game as I will explain.
In my opinion Halo 3 is an amazing game, its in the top 3 games ever made in you ask me. The experience is fantactic, graphics good, gameplay good, storyline good and multiplayer is exceptional; its been the top game played on xbox live for the past 3 months and has been the top played almost every month since its release!
Halo: ODST looks like it will be as good as Halo 3 and more!
It was originally meant to be just an expansion on the oringinal but Bungie found they had so many things they wanted to add to ODST that they had to make a whole new game, retailing at approximately £40.00 in the UK (I believe that to be about $60.00). It is gonna have a whole new stroyline extra maps for Halo 3's multiplayer, a new look, new feel, new everything. I can guarentee it will be even better than Halo 3..
If you want fun buy Halo 3, it is by far the most fun game I have ever played and I know a whole lot of people who feel the same way; considering its now dropped in price its more than worth it. When you've finnished with Halo 3 I suggest you start saving for ODST cause this looks like it will be one awesome game.