I started watching it from the beginning, for two reasons:
1. The Premise sounded cool (an entire day in real time).
2. Keifer Sutherland is one cool fugger. Dude is a drinker, smoker, cusser, etc. Pretty much the anti-Hollywood regular guy.
Now I just stick around to see how many twisted ways he kills people without predjudice. Excellence of execution, if you will.
I don't think I'd start with season six. You have to start at the beginning to see the evolution and the fact that Jack Bauer has killed more people than smallpox.