It has to be said that 24 is the best television show on earth. There are other television shows that I love (i.e. Futurama, Simpsons and of course the Who) but nothing in the world can beat 24. The production, stories, acting and cinematography of the show is unlike anything on television. It has to be said that the first season was a little slow and not so interesting, but it was deffinately essential as it set out the style of the show and essentially set the pieces for the rest of the series - ones that just get better and better. Each season is amazing for its own reason(s), and for those reasons the seasons keep improving, though in my opinion season 2 was the very best.
One thing that should be noted about the programme is the reccuring appearances of characters that have featured before but then take breaks for a series or two, and then return with new objectives and character styles. It's a testiment to the actors and casting directors for what they bring to the show.
On the other hand, the introduction of new characters with high profile actors is also fantastic, because the increase of acting talent means that the enjoyment of the text is greatly enhanced. Of course, the only way to pull in good actors is to gain a good reputation as a show, and this is proof of what 24 has achieved in the progression of the 6 series so far.
I'm currently watching season 4 of the show, the only one I haven't seen so far, and it has reminded me of how outstanding the show is; whether you're watching it for the first time or for the fifth!
If you can, get a hold of 24 (best to start from season 1 but anything will do) and get watching, trust me, you won't regret it :)