Does Saint John's wort relieve depression?

Short answer: Yes!

Longer answer:

• In one study it was significantly better than placebo for mild to moderate depression and similar to tricyclic antidepressants. [Linde K, Mulrow CD. St John's wort for depression (Cochrane Review), In: The Cochrane Library, Issue 1, 2000, Oxford]
• It has been found effective for SAD (Seasonal Affective Disorder) [J Geriatr Psychiatry Neurol. 1994 Oct 7. Suppl 1:S29-33)(Schweiz Rundsch Med Prax. 2000 Dec 14; 89(50): 2123-9]
• It was effective for major depression, being better than placebo, similar in effect to conventional medications, and having fewer side effects than conventional medications. [Cochrane Database Syst Rev. 2008 Oct 8;(4):CD000448]

There are many many more studies on the effectiveness of this herb - too many to list here.