I don't know if it's placebo or not. There have been some studies done that give evidence that St. Johns Wort actually does work. I think it's one of those things that just effects different people in different ways.
What it has done for me is to give me the ability to feel again (or so it seems, anyway). For a long time I felt like everything was devoid of emotion except for anxiety and sadness, and there was no motivation to do anything. St. Johns Wort almost feels like an emotional lubricant, and gets the wheels turning again.
That having been said, it hasn't done a whole lot for my anxiety.