NB: Spoilers within. This great movie is "about" so many things, all of them successfully: sci-fi time travel, unstable psychologies, dystopian society, the what-is-real syndrome, gradual undermining of belief systems, worldwide bioterrorism, and a nascent love story.
The ramifications of the story's twisted time line stir up loads of heated debate - witness the discussions within this site; or, as an extreme, check out the dissertation at www.mjyoung.net/time/monkeys.html. Whew! Such temporal emphasis speaks mostly to the brilliant plot, coming from the magnificent work of writers David and Janet Peoples, not to mention the inspiration of Chris Marker's "La Jetee." Without a doubt, this is one of the most successful, fascinating time-travel movies ever conceived. But there are many other levels speaking here.
The movie's real genius is to focus on the nasty side effects of time-travel in the mind of James Cole (Bruce Willis, doing the best work of his career here). His journey progresses from gung-ho vaccine-hunting warrior to gradually unhinged victim  and back again. The other broad sweep of the story increasingly emphasizes the personal tale between James and Dr. Kathryn Railly (the wonderful Madeleine Stowe). I love the simultaneous shifting/opposing viewpoints of these two characters. For me it all comes to a head in the fleabag hotel room scene. By this point, James  once gripped by an unshakable determination  now slumps in utter doubt about his own reality; while Dr. Railly  the cool and rational scientist  has finally become wildly convinced, after absorbing James's proofs, of his horrific predictions. Her desperation to get through to James and hang on to the mission shows how far she's come.
Gilliam makes us care about these characters, especially through the crescendo of tension threading their lives. The balance held between emotional roller-coaster and mounting sci-fi puzzle/thriller is exquisite. And the denouement at the airport is heart-poundingly intense because we see it coming so clearly through James's dreams. It is here, just after James has decided to quit the whole mess  and is fighting his insanity more than ever  that he steps back up to the plate and does what is necessary for mankind. See Jose and the gun (Just before this, the references to Hitchcock's "Vertigo" and identity switching/confusion are brilliant.) This is a movie to be hashed out between thinking people; it not only holds up under repeated viewings, it demands them. "Twelve Monkeys" is intelligent, provocative, bizarre, funny, and suspenseful stuff.
The supporting cast is excellent, especially Brad Pitt stealing all of his scenes and showing great flexibility as Jeffery Goines, crazed and spoiled, but ever the survivor. And there is David Morse as Dr. Peters (interesting how the movie simply leaves to the viewer his wicked motivation) and Christopher Plummer as Dr. Goines. But the biggest accolades belong to Terry Gilliam, surpassing here - just barely - his outstanding "Brazil." (Lots of parallels, of course, especially the lonely combatant trying to escape his crumbling surroundings: lunacy within, lunacy without.) Every frame of this movie has his unique stamp and tone. The soundtrack is terrific, too.
This is one of the great achievements of the 90s, a true favorite of mine, and sure to hold up for a long time to come.

Even for the cocaine laced 1980's this is a pathetic. I don't understand why someone would want to waste celluloid, time, effort, money, and audience brain cells to make such drivel. If your going to make a comedy, make it funny. If you want to film trash like this keep it to yourself. If you're going to release it as a joke like this: DON'T!!! I mean, it was a joke right? Someone please tell me this was a joke. please.