The first series is sci-fi at it's absoulte best. It was also pretty damn funny. The hero is not your normal guy: he's a messed up alcholic. But every single episode was wow. When the events of 9/11 transpired I was thinking about the pilot episode and how similar these events were. I don't think the show could have run on without confronting the issue so it's just as well it ended when it did. IIRC it got canned due to Buffy being brought.
The second and third series' aren't too good. Just good solid entertainment. Very funny at times too. But it holds nothing on the first series. Still I've watched them all and still have around 8-9 eps to go of Series 3.