In the UK there are some doctors that actually prescribe it to patients instead of your regular drugs... but only for mild to moderate depression. Usually the dose is at around 300mg * 3 doses, spread evenly throughout the day.
As far as I have read (and I've read quite allot on this), the studies on its effectiveness are not as strong as for the drugs themselves, but very convincing none the less.
I personally used it, but only for a month. I did notice I didnt feel as anxious or depressed... but Im not sure if that was a natrual progression or the St John's itself. It is meant to take approx 4-6 weeks to work. I only took it for four. I do know quite a few people who swear by the stuff when their down in the dumps.
Another plus side is it is easily affordable.
If your interested in other herbal anti-depressants run a search on Sam e and 5htp. Hope that helps.