THE SHORT: A solid shooter with great mission objectives, fantastic multiplayer, and lots of options and customization. Don't care too much for the alien plot, but love it otherwise.
THE LONG: To me, PD is the best shooter ever. I'm not one for the more sci-fi, "kill aliens on other planets" shooters (though I admit that at least a portion of this game is spent doing just that).
No, give me a shooter where I outwit actual human beings who look like human beings, with actual red blood. Give me three dozen weapons with ingenious strategic attributes that makes using each one unique. Let me enjoy the game at home, by myself, by playing against up to 8 other computer controlled characters while my friends are unavailable to play against me. Let me create my own scenarios, choosing the weapons I want and with the limits I want for all-out melees. And while you're at it, throw in 50 cheats that drastically alter gameplay. I love it!
While the story of PD is pretty throwaway, and the voice acting only ok, the real meat of this game is how fun it is to play. While you do need the expansion pak, there is a great 1 player game as well as a multitude of multiplayer options that allow you to customize every little detail of your vs. battles, whether going against friends or the computer itself. The framerate is usually pretty smooth too, with pretty graphics and shiny guns blastin' away. Sound effects are loud and great, and the music is fitting as well.
I honestly can't think of anything I really dislike about this game, and having played Goldeneye extensively, I'll never know why PD doesn't rank higher. (EDIT: I guess at least it sometimes does, depending on votes). In my opinion it's just a better, more polished game, and I think it will always be the FPS closest to my heart.