I do love Supernatural, though I agree that some of the episode plots can be a little hit-or-miss. Whatever you do, though, don't watch either of the vampire episodes ("Bloodlust" and "Dead Man's Blood"), they completely butcher every vampire myth ever. I think there are quite a few good episodes in seasons 1 and 2, and I'd start there if I were trying to get someone into the show.

Have to agree about "Lazarus Rising". . . not an uplifting episode, to say the least.