I got it yesterday, and played it for a few hours before losing interest. Granted, I've never really liked the other games in the series, but I really don't understand the hype. It must have some pretty smashing multiplayer, because single player is nothing special.
I don't want to start an argument here (I know Petrarch likes the series), but I simply don't understand what is good about Halo in singleplayer.. I can't think of a single thing that hasn't been seen in some other game already.
Halo did quite a few "firsts" (for a mainstream game, as some of these ideas may have existed in some form before)
* the regenerating armor (and later, regenerating health) - totally changed how FPS handle healing.
* Easy to use grenades - no more switching your weapon to 'grenade' to use one. You could (*gasp*) hold a gun with one hand and throw a grenade with the other. Made fights awesome.
* Incredibly varied, simple to drive vehicles (that were a blast to use)
* Excellent co-op. Fantastic Multiplayer.
* A biggie - every gun was useful, all game long.
* 2 guns at a time. That's it. I love this.
* Decent enough story. Ok, it's not the end-all, be-all, but there's WAY more story, and better developed characters, than 99% of shooters.
* Incredibly varied environments and battlefields. Yes, the whole "cut and paste" thing is a complete myth that console haters love to grab. Show me any shooter, PC or console, that doesn't have similar looking environments from time to time. ANY?? And in Halo, it made sense - you go in, you get out. It would make sense that the hallways of a huge building look similar. That said, there are some seriously good battlefields in Halo (fighting your way across the bridge? Driving the tanks on the snow and ice levels, etc etc)
I've played a ton of shooters. I've played shooters since the very beginning of shooters. All things equal, there isn't any shooter as much fun as the Halo games. They just have that certain "something". Some shooters I've enjoyed as much for a short time (SS1 & 2, HL1, Doom2 and 3, Duke Nukem, Call of Duty, Far Cry), but none of them are anywhere near the game that Halo is, for the reasons mentioned above.