Half-Life

The Good
The scientists/guards talk to you, and help you. The game has a really scary atmosphere to it (play it late at night with the lights off). The weapons are realistic (no BFG 15000's here), you find ammo and weapons in realistic places like on bodies and in gun racks (not hovering above the ground). It's quite a challenge too, so you'd better save often. The multiplayer function runs pretty smooth even if your ping is terrible, and the multiplayer modifications for it are really impressive.

The Bad
At certain points it gets too damn hard, makes you want to cheat. The multiplayer community (like all gaming communities) has some idiotic people in it. You need a top of the line video card to play it optimally.

The Bottom Line
Not another DOOM or Quake, should be in it's own genre. If you've ever wanted a FPS that had more behind it than "Save the earth", then get half-life.