It's a great game even up till now.
Combat Evolved raised the bar for storytelling in video games with plot twists and mysteries unraveling themselves as you progress and a feeling that you are on an alien world. It also has some humorous dialogue.
The gameplay had a few innovations. It introduced the two-weapon limit and a [sort of] balanced weapon set. The gunplay is fun, too.
But the game has little replay value, as once you are done, you don't feel much need of replaying the game very soon. Maybe you would for a level or too. But levels such as "The Library" are over repetitive with copy-paste corridors and will bore the life out of you. Other levels are identical to each other. Assault on the Control Room and Two Betrayals are identical. Truth and Reconciliation and Keyes. The Pillar of Autumn and The Maw are among these.