Each season is a new story arc that, while it makes references to the past seasons, runs well enough on it's own. Most of the other players besides Jack Bauer (Keifer Sutherland) change so much between seasons that it doesn't matter.

And each episode starts with a recap of the previous, so you should be able to jump in while the season's young.

I haven't followed the subsequent seasons religiously, but the first season gripped me from start to finish. I had more time on my hands then. From what I've caught of the other seasons, they've maintained the quality and suspence... True, at some point, you start to question the likelyhood of one man being at the center of (and having the wherewithall to difuse) 5 successive major national security crises... But other than that small detail, I'd recommend it.