Brief History of the Halo Universe
I like Halo. That's probably not a shock to anyone - in fact, you probably expect a love of Halo to be a part of the Microsoft indoctrination process. That said, I really do like Halo, even without the brainwashing. I'm not a big fan of the multiplayer (I know, I know - heresy!), but I love the story and Halo mythology. There's just something epic about the Forerunners and the hints you glean about them as you play the game that's completely hooked me, and I'm looking forward to finishing the story arc with Halo 3.
The game isn't perfect. In fact, I'll expose myself to the Microsoft snipers and say that in many ways Halo is over-rated. The weapons are good, but nothing mind-blowing. Gameplay is a lot of fun (and extremely well-balanced), but man... can it be repetitive! I just finished replaying Halo 2 (on Windows) and was reminded of this when I went through the umpteenth cloned hallway and walked through nigh-infinite series of ambushes. <sigh> And for the sake of not breaking my keyboard against the wall, I won't even mention Halo 2's non-ending.
All that said, probably the most frustrating aspect of Halo for me is the wonky audio mixing. As I said, I just played the single player game of Halo 2 through to remind myself of the story and try and catch aspects I'd missed before. Unfortunately, even the second time through there are many places where Cortana tries to speak but you can't hear her clearly, or one of the sentinels (Penitent Tangent in Halo 2) hints at backstory but you just can't catch it. It's incredibly frustrating, but luckily someone pointed out a great Halo Universe "bible" that you can read to get a great summary - including integrated details from the novels.
Take a read if you've never "gotten" the Halo mythology. And do check out the games if you haven't... they're a ton of fun, especially co-op. Frankly, the most fun I've ever had on a console was playing through Halo 1 and Halo 2 on the Xbox with a friend on the couch. Good times!