Series 1 is somewhat "monster-of-the-week" and there isn't really an overarching plot, with the exception of a few episodes. Series 2 improves on this slightly, but each episode is of a much higher quality.
If you've seen Captain Jack Harkness in Doctor Who, then you already know how awesome a show based around him would be. If you haven't seen him, then go see him. Then watch Torchwood.