I am new to the show, having missed most of the first season except the last two episodes, but I read the back-story in TV Zone magazine before watching it. And I realized I had been missing one of the best shows that has ever been on television.
Okay, it's not as good as Millennium, but we needn't always compare the two as they are quite different in tone, if not in concept. In the few episodes I have seen (on the NBC Superchannel) I already know the characters very well, which shows the quality of writing on the series. There is also a continually developing storyline that is great to watch unfold - most shows don't bother with as strong a continuing arc as this one, fearing that ratings won't hold, but this show keeps the episodes themselves self-contained while including the developing Jack arc. It's very well done.
Well, all I have left to say is that, if you haven't seen it yet, please do. Okay?
Links:
Insight:An Unofficial Profiler Page
Celli's Profiler Page
Only You, Only Me
The Believers
There are, of course, plenty more sites, but I'll let you follow the trail...