I remember seeing the first teaser-movies for the first Halo, before it was even taken off the PC development platform and made the 'killer game' for the Xbox...that was about 8 years ago and Halo 3 is the climax of those 8 years of enjoyment. This is not an understatement - geekish self-indulgence aside, this is one of the most simply-stunning pieces of entertainment. I played Halo in my first year at Uni on a completely co-op basis with my corridor-mate and it was one of the most fun gaming (and, shock horror, social) experiences ever. I only played Halo 2 on the 360 having realised I hadn't actually gotten around to playing it at all, and then when this came out I had high hopes...and I was not disappointed.
Games likes this are extraordinary - there are really only 3-5 games like this in any genre on any platform that can cause such fondness of memories for people to indulge in a story so much. Metal Gear Solid (and its sequels), Half-life (and its sequels), Gears of War (and its sequels) and of course Halo are probably the 4 that come at the very top of the action/FPS genre (I can't count Call of Duty as the first 2 were great, 3 was a waste and Modern Warfare is brilliant but too short - stick with Infity Ward and the MW layout on this, that's all I'll say)...
All four of the above games have a rich storyline that the game seems to THEN be built around - building a game and then fitting story almost never works. Not only this but the graphics are stunning and a new-generation take on how things can be done. They are voice-acted superbly and time is spent on making characters real, rather than just lots of tiny parts and one main protagonist (see the brilliant, massive but flawed Oblivion: Elder Scrolls!). However, one thing that really has struck me about Halo 3 and above and beyond the other games mentioned, is the musical score. I think music is such a hugely under-appreciated tool for drama, emoting from the player and generally making scenes of quality in to scenes of spectacular story-telling to the point of spell-binding. I completed this just a few days ago (been distracted by other games!) but I keep re-thinking over the musical parts and some of them are, I am not afraid to say, genuinely emotional - but again, this can only work with a story-arch you can utterly invest and believe in (to some extent, this is Sci-Fi).
The single-player is well structured and of decent length (although I think slightly shorter than the other predecessors in the series), the graphics are sharp and soft in different respects when needed to be, the game is manageable from a difficulty perspective, the multiplayer is varied (although challenging at times) and it's just generally a game that has left me quite stunned. Not since I completed Half-life2:Episode 2 was I left wondering what/if there will be a sequel and re-counting the different moments you remember throughout the journey.
This is a special, special piece of entertainment; a line in the sand for the pure combination of factors that must be brought to bear on all games that deem to raise themselves to the level of quality that surely all designers want? There is a reason why people are quietly murmuring about how gaming is becoming a newer and deeper platform for entertainment expression, even rivalling film - that reason is games like Halo 3; that reason is care, attention and pure skill at design and craft; that reason is the passion that gets transferred from designers to the consumers. I was gutted that it had to come to an end at some point...but you can never quite trust Bungie - they always manage to surprise and they could well continue this beyond Halo 3...
Ladies and gentlemen, please enjoy one of the finest pieces of interactive story-telling you may ever have the fortune to experience... :)