I would say Halo is easily one of the best FPSs ever made. While I don't quite consider it the best ever made, I certainly think there is a very strong argument to be made that it is.
Visuals on the game are quite good, it isn't HL2/Doom3 but it still compares quite well with the overwhelming majority of titles out today. Gameplay wise the title is exceptional. There are several factors in the FPS genre that Bungie played with(and GearBox kept in tact) such as only being able to carry two weapons, always having grenades on hand and being able to pick up any weapon that isn't grafted in to an opponent and the ability to take control of various craft while playing the game(this is becoming more common today of course). The enemy AI in Halo is second to none at the highest difficulty settings. The only real issue with the game is some of the levels have a repetitive feel about them in terms of level design, even Half-Life had a rather serious issue though(Xen).
If looking for reviews on the game, you can check out the XBox reviews and keep in mind that the framerate is better and the controls are improved significantly over the console port. There are some minor changes in things like the cutscenes over the XB version, but other then that it is the same game.
Edit-
Forgot that they did take co-op out, but they also improved on multiplayer.