While talking with E! Online, CW president Dawn Ostroff talked about the possibility of extending Supernatural to a seventh season and it looks like chances are really good:
"We haven't officially picked anything up yet, but [Supernatural] is creatively doing so well—and it's doing better on Friday night than it was on Thursday night—so I can't imagine we won't pick it up."
I love Supernatural, but it is one of those shows that hardly any horror fans I talk to have seen. It took me years before I started watching it and I think most horror fans had the same wrong assumption about it that I did. I saw two young male actors on a show on the CW and expected to see a cheesy ghost show aimed at teenage girls. Instead what you get in seasons 1 - 5 is an extremely well done pulp horror show pulling from american ghost myths. It starts out a little slow and formulaic in the first season, but it definitely pays off as you keep going from season 1 to season 5.
As for season 6, I haven't seen anything yet to convince me that the show shouldn't have stopped at the end of season 5 as it completed the story arc started in season 1, but I'm up for them to prove me wrong by the end of this season and hopefully with season 7.