I actually followed Halo 1 for about 2 years during it's development cycle, back when it was going to be a Macintosh exclusive. I watched it morph from an open world, 3rd person strategical shooter (you could go anywhere on the Halo, there was even wildlife and such) that was aiming at 40v40 online multiplayer games, to an xbox exclusive FPS. They completely changed everything about the game and that made me sad.
Yet still I've played Halo 1, Halo 2, Halo 3, ODST, and even tried out the Halo Reach beta. And I have to say that imo the series is overwhelmingly mediocre. Amusing for a time but not at all worth the hype. Also, Halo ODST was incrediboring