The TV show is loosely based on the 12 Monkeys movie from 1995, but most of that relationship is in the initial setup and character names. Once the series gets going, and particularly after the first season, it bears little resemblance to what was in the movie. Not surprising, since they had a lot more hours to fill.
Time travel is always a difficult beast to pull off, but 12 Monkeys did a fine job of it. The ability to travel was limited by technical issues and potential for paradox, so the viewer could believe the difficulties that the characters were having couldn't simply be solved by going back a little further in time. And the limited information available after the plague made it more believable that it wasn't possible to easily eliminate the source of the problem. Later on, some of those limitations were removed, but by then the problems were greater and the enemy more capable. On the whole, I think 12 Monkeys pulls off the time-travel theme about as well as can be expected.
I enjoyed most of the characters in the show as well, although the leads were far from the best in my opinion. Both James Cole - handsome rogue from the future - and Cassandra Reilly - implausibly beautiful and brilliant doctor - felt too "TV perfect" for me to really have much empathy for them. Sure, they were in some crummy situations, but of course they always found a way out. But several of the supporting cast were great, in part because the characters actually had some believable flaws. By far the best character as far as I'm concerned, both in terms of acting and writing, was Jennifer Goines played by Emily Hampshire. She played the driven-insane-by-warped-time character perfectly, and the writers gave her some of the most hilarious and memorable moments in the series.
One of the most difficult things to do with any multi-season series is a really satisfying ending. On this score, I give 12 Monkeys high marks. The way that they wrapped up the plot was excellent, covering pretty much every major question that had been raised along the way. I'm sure there were a few minor things that didn't fit, but there wasn't anything that bothered me enough to matter. The ending for the characters was a bit too good to be true, but I really wasn't expecting anything else by that point.
All told, I'd recommend 12 Monkeys to science fiction fans. I'm not sure it'll draw in too many who aren't interested in the time travel premise, but it's a solid offering for those who are.
