HALO is one FPS that really did introduce many new gameplay elements. It offers all that is said below and the modifications already in development look to refine and improve the "HALO experience" even more!
This review may contain spoilers
This is Halo: Combat Evolved, It will take you through a diverse universe that is later expanded by it's sequels and other materials such as novels, movies and short stories. The challenges in this game will be depending on the difficulty you chose. (I suggest heroic for the best experience). While Halo: PC is the port of the original Halo: Combat Evolved for the original Xbox, it suffers due to various technical things like the low FoV (field of view) which is exactly 75 as opposed to the original game, which is 90 and various missing graphical shaders i.e.: The Jackal shields on the original Xbox version of the game are either blue/orange and has a "damaged" graphical effects if you shot them with a plasma weapon. While missing various things from the original Xbox version, it is still Halo and they even added online Multiplayer as opposed to the Xbox LAN only Multiplayer as well as some other neat additions not available on the original Xbox game. But this game has an awesome storyline and gameplay back in the early 2000s and this is the game that brings you ultimate challenge in Campaign and Multiplayer.
That's all I had to say, and that's also why I gave this a 10.
If you played this back then in the early 2000s, you probably are going to have some nostalgic memories on this game and I liked to play this game back then up to the point that this is the game I had been playing the most.
P.S. Get Halo: Custom Edition after getting the PC version of the game, tons of maps and mods as well as active Multiplayer community. You can mod the game like I do back then and make some neat AI war, with the Editing Kit available for download if you have it.