Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
By far the best and most fun game I have ever played. The game is packed with innovative features that even most FPS today fail to live up to. And the level design is exceptional, the ambience is fantastic, and the music is brilliant. This is the game that pioneered co-op and bots, and secondary functions for all 50 or so guns, and easily still has the best and most customizable and user friendly multiplayer I've ever seen.
The storyline is quirky and interesting, if you were expecting an epic saga in a game like this you are looking in the wrong place. It's not brilliant but I didn't really have much complaints. It was the first time I had ever seen a flying, talking laptop with eyes.
The gameplay is sensational, provided you enjoyed GoldenEye type FPS games, just going forward and shooting everything in the way, with some gadget interface and hostage rescue, basically a wide variety of objectives (again, considering the release date). The guns are phenomenal, the noises they make firing are great, the reloading animations are great, and they are all innovative. I don't think I've ever seen guns have secondary functions in a game before, Perfect Dark has to be the first, and they are brilliant additions. The Dragon can be used as a proxy mine, the Phoenix can shoot exploding orbs, the Reaper can work as a chainsaw, the shotgun has a very cool double blast, the CMP150 can lock on to targets, the Slayer has a fly-by-wire rocket (a first as well, very fun in multiplayer), the RCP120 has a cloak function, the Laptop gun becomes a turret, and there are many more, and several GoldenEye guns can be unlocked too.
The replay value is endless, with a co-op mode (again, I believe the first game to come up with this) you can play with the computer (if you are an only child or have no friends or whatever) or a friend, and you can unlock different and better bots than the standard blonde bimbo, such as the Alien, the dude with the revolver, and some others. Also there is a cool Counter-Op mode which I haven't seen anywhere else, where your buddy is a bad guy and he has to stop you from completing your objectives. The incredible multiplayer also adds a lot to this. Also the tutorial area, or the Carrington Institute, is very well made and you can spend ages at the firing range or gadget tutorial, or the maze, or at the information terminals for a good dose of sci-fi.
The level design is brilliant, for both indoor levels like the DataDyne Central: Investigation level and the outdoor ones like the Skedar one, the Crash Site, and each area is distinct and very well made. The graphics were as good as anything N64 ever displayed, especially with the High Res option.
And finally, the multiplayer. This is as good if not better than the story mode, and you don't need real, living friends to play with you. Every possible option you could ever dream of customizing in multiplayer is there. There's a hundred head and body types, (an alien head on an office lady never fails to amuse), you can choose how many guns a level can have and which specific guns, there are about 15 arenas or so, including some GoldenEye classics like Facility, and there are several modes like Capture the Flag, King of the Hill, and a ton more that I can't really remember right now. And the bots! About 20 different types of bots that you can customize, make your own, have as allies, enemies, whatever you want. It still amazes me that not too many games have picked up on this. You don't have to wait for your friends to come over for a good, no-holds-barred, shoot 'em up. Additionally the challenge mode in Multiplayer is brilliant, another campaign on its own, with 30 different challenges encompassing most of the multiplayer options.
Basically it's a must-buy, a Holy Grail for all FPS games, and it still has the best and most fun multiplayer, with Halo not too far behind (but without the bots or too many customizable options). And this is why I absolutely abhor Perfect Dark Zero as well.