ODST's story was amazing, gameplay wise it was kinda lacking. You're not playing as a walking tank but you get to experience what marines and shock troopers had to deal with against the covenant. I really didn't like their approach with swapping characters throughout the game (sorta confusing when you jump from one character from another after they're done their part) but they nailed it storywise, Rook getting lost and finding pieces of his teammates equipment and eventually linking back up to them was very nice imo.
If they bundled it with halo 3, it would have been very nice. They really didn't have to make a game for it but it really expands the halo lore and ODST focuses more on the marines side of Halo while 1-4 focuses on Chief. I think they did okay with ODST as hell jumpers are pretty badass in the halo universe. Honestly if they actually had at least one of them drop on the covenant carrier it would have made the game 100% better.
ODST was a very memorable game story wise, the gameplay was forgettable as it didn't have a good multiplayer, just endless waves of enemies thrown at you.