Rather than blocking the reuptake of serotonin, St John's Wort tries to just promote serotonin generally. As far as I'm aware there is some scientific evidence that this can be effective in mild depression, but so far it has not been shown to have an effect in more serious cases. Before beginning my SSRI I tried SJW for a whole year and honestly, it did next to nothing for me as my problems were not mild. My partner seems to swear by the stuff for a mild case of the blues though. Give it a go, but really, I highly doubt it could replace an SSRI for those who really need one.
Glad to hear it, and eggsprog has good advice, although if you've read your info leaflet you'll know that already