I'm up to 2X08 "Crossroads Blues" now. Season Two is MUCH better than One. It's definitely a lot closer to the show I know. I was pretty surprised that the daddy died so soon in the show; so far Bobby has only appeared once (in 1X22), I love that guy. A lot more interesting than the dad was!
Looking back on it, I think I basically started watching the show in Season Four, because I don't think I saw episodes before Castiel appeared. I might have seen the occasional tidbit before that.