Probably the best crafted sci-fi series I have ever seen, maybe even better that BSG. Started out as a longer version of the Bruce Willis film, evolved into a story about the role of time travel and causality and finished off as a love story.
There were loads of threads that were spun throughout the series. Some seemed to be random asides but they all ended up being relevant, adding to the story. Every single loose end was wrapped up in an unforced, satisfying way in the finale unlike other shows (like Lost) which lost their way.
Sad to see it go. Definitely one to re-watch at some point.