Halo is one of those "had to be there" experiences.
I'm probably one of the few people on the planet who's managed not to play Halo all this time, which is really too bad because even after all these years, the game holds up quite well. It's more of an appreciation in hindsight, as Halo has obviously been eclipsed by modern shooters like Call of Duty 4, Half-Life 2 and Crysis. Open-field battles, two-weapon fighting and regenerating shields were really popularized here first and still make a great impression.
The game world borrows a lot from action sci-fi like Aliens, but manages to establish a dramatic mood with the fantastic score and fights that build up in momentum through levels. The game definitely plays up your ego as NPCs continuously make comments to you like you're a savior and battles often make you feel like you're coming to the rescue just in the nick of time.
Unfortuantely I was only able to play about half the game before graphic problems kept me from progressing. Game freezes on a game this old are pretty ridiculous ... FEAR had the same problem with a certain point in the campaign just stopping you from progressing b/c the game would just lock up at a specific location.
I got the game used for dirt cheap, so not a huge loss, but just a warning if you're paying full price for this -- maybe you'll be more patient and willing to try different graphic drivers to get past the problem.
I enjoyed what I saw and basically got my fill of Halo. Ah well.