An excellent First-Person Shooter with it's faults, but is certainly an enjoyable game. Overrated? Perhaps. Good? Yes.
But is it?
As a PC-only gamer, I enjoyed the Single Player Campaign for Halo: Combat Evolved, though I have not tried the multiplayer. I think it is a tad bit overrated (not deserving of the 9.7 that it received from Gamespot) but I would not call it "mediocre" or "average".
What are the faults? Well, it has an uninteresting "Space Marine" techy character with no personality. Master Chief seems to be the type of guy who follows orders around like a robot. His backstory isn't even explained in the game, and game tries to make him "kool" by not revealing his face.
It has repetitive level design that involves a lot of back-tracking. Often, each map you will feel like you have been through the same corridors over and over again, and in some of the levels you'll be moving back and forth.
It is poorly optimized.
And the weapons of the "good side" are generic and boring "Modern Earth-like" weapons used by the UNSC. From the "Pistol" to the "Assault Rifle" to the "Rocket Launcher" which are pretty common FPS weapons only without interesting names.
The Halo Universe itself is interesting, though little of it is fleshed out in the game. Master Chief's background, as stated, is not revealed in Halo: Combat Evolved - nor is much of the background of any faction in here.
However, there are good sides to the game. It is good if you want some "simplistic" fun with maybe a bit of thought involved. You can find yourself sneaking around while cloaked and wondering which enemy to stick a plasma grenade on.
You can slowly and quietly walk up to an enemy and whack him on the back with your Shotgun. You can choose to rush in an area full of Grunts using an Assault Rifle and let your shield take the brunt of the damage, while lauging evilly as your bullets cut them apart.
The enemy weapons are interesting and varied, from the charge-up capable Plasma Pistol to the explosive spike-throwing Needler. Covenant Plasma Grenades are like sticky bombs, except that you throw them and they attach to organisms as well, not just vehicles.
Enemies, too, are varied and interesting. You will be fighting enemies such as the puny, weak but numerous "Flood Infection Forms" to the cowardly "Covenant Grunts" and the walking tank "Covenant Hunter" which requires you to kill it via weakspot blasting.
The sound is okay. But the music is astounding. The Halo theme song is one that is worthy of an orchestra, and it certainly sounds like it's made by one. It is used for the game's large-scale epic moments at the correct time, and the effect can be quite dramatic.
The graphics are okay - Physics are fairly up to date, with weapons and NPCs flying around whenever there's a boom to throw them. It's certainly up to standard for a 2001 game. Not as good as say Max Payne 2s, but still acceptable.