I bought this whole box set without having seen a single episode -- so strong was its reputation. And we haven't been disappointed. It's been one of those rare TV series that both my wife and I have enjoyed watching together.
I would agree with the general view on this forum that Series 2 is the weakest of the three. I think that's largely because the two threads of the story -- the nuclear threat to LA and the father's threat to Kim -- stay completely apart throughout. In Series 1, every thread contributed to the overall story. Series 2 finishes particularly unsatisfactorily, with the master crooks revealed to us only minutes before the end, and no resolution there, or at the start ofSeries 3, to tell us how they were captured.
Series 3 has two largely unrelated stories -- the presidential re-election and the viral threat to the USA -- but it is somehow much better done than Series 2: it's clearly got a bigger budget (for two jet fighters and a cast of hundreds in the hotel and the metro stration), and the music is better. Every key character compromises themself in some way or other -- even the President. The only problem with Series 3 is that the entire story in Mexico could be detached from the LA outbreak story that dominates the second half. And the fake courier plot that starts off the series seems largely irrelevant to everything else. Constructing a 24-episode day is clearly a very difficult thing to do.
I can't help thinking that the central character, Jack Bauer, is a US attempt to create a James Bond equivalent. (Maybe I'm being deluded by the initials.) But it works well.