[quote name='Nintendokid54']Is this worth it if i have ODST?[/QUOTE]
Depends. If you only really care about multiplayer, then no, because ODST has all of the Halo 3 multiplayer on the 2nd disc.
However, if you want to play the Halo 3 campaign and want a pretty cool Master Chief helmet, then yeah. The other great thing about the Legendary edition is that it comes with a movie of all the cutscenes from Halo 1 and 2. It is perfect for someone who doesn't know the backstory, and is great for those who played through both of those games, but may have forgotten some of the storyline.
For $40, as long as you're interested in playing the single player, I'd say go for it.