If you're a hardcore Halo fan then you'll probably critique alot of things about this game.
Personally, I didn't like the story, the cinematics, or the voice acting. It was all a little cheesy and cartoony. The whole illusion of romance between two of the characters was was not taken very seriously and seemed like a last- minute story tweek. The cinematic graphics were somewhat choppy looking and the voice actors were amateur sounding.
Although the story has changed drastically, the gameplay is very similiar. All the same weapons are included with the exceptions of a few.
(Battlerifle gone,Sword gone, SMG changed, Pistol changed)
Playing as an ODST limits your jumping and health regeneration. And for some very odd reason the ODST can use any vehicle except the wraith, which struck me as very odd. Why go through the trouble to change something that would aggrivate people?
Other than the flaws I stated above, progression through the game was fairly enjoyable. Four player makes the campaign fun. The firefight mode prooves challenging and entertaining.
(Tip: Co-op is extremely easy with four people due to increased respawn rates)
In conclusion, the game is worth buying, although it really is an odd addition to the Halo series. If the game isn't what you expect, than at least you have Halo 3 multiplayer and maps out of it. Or you could just rent it first.