Halo 3 is the last game of the series. They say it wraps everything up. However there are 5 or so books that tell you the whole story before Halo: Combat Evolved. Most people don't like Halo because either they don't get the story having not read the books or played Halo1. Though a lot of people have said its too hard because there's no smart scope/auto aim to help you shoot enimes with accuracy. I like that part.
Makes it a challenge.
And if you don't have X Box live, brothers, or friends to play Halo 2 with its not so great because it was made for multiplayer.