Moodscope could be a useful tool for depression sufferers and is certainly worth considering as a part of a support network, as long as your social and psychological profile fits its audience profile.
I have some concerns, however. Although the founder's hope is that it will raise awareness of depression as a serious and common issue that is frequently marginalized by the British health system, it may also have the opposite effect. Major depressive disorder is a serious medical condition and may require deep, persistent professional treatment; but there are too many skeptical people who will look at this site and take it as proof that the illness is something that can easily be cured without any professional intervention, just by filling out a few flashcards. And that's something to be avoided at all costs, since any chance to minimize the seriousness of mental illnesses tends to lead to less care and less belief in the existence of the illnesses in the first place.
I don't wish to take anything away from the idea or the execution, and if people have supportive friends - by no means a given, in depressive cases - and are not in urgent need of professional medical or psychological support, this could be a big help in learning to understand the nature of their own mood swings.
Many depressives will also become dependent on anything that seems to offer help of relief, and I feel this site should take itself more seriously and recognize that this is not just another area in which to develop a commercial internet scheme. If it becomes successful, it will have a responsibility to the people who come to trust, or place their hopes, in it. Part of that responsibility is to make sure new arrivals see a message advising them to call the emergency services if they are having suicidal feelings, and that the phone number of at least one voluntary helpline, perhaps the Samaritans, is there in plain view. Another part is to offer some help and advice to those for whom the site doesn't work at all, and who may become more depressed as a result.
I am also concerned that not enough thought has been given to accessibility. The front page offers an explanatory video clip, nothing more. Almost everything else is presented by way of cool, web 2.0-style minimalist animations. None of this is even visible to people using browsers which are customized for accessibility. Depression and mood issues are not confined to those who will see and appreciate the design style of the site more than the content and older and more challenged members of the community are particularly vulnerable. I don't think enough consideration has been given to a wide enough target audience here.
It's all good, as far as it goes, and anything that helps is welcome. But there are already mature resources online, and communities which function as self-help groups. This needs to be bringing something new to the table aside from what is basically a gadget with smart graphics, in my opinion, and be very careful not to minimalize the dangers of the illness, nor target too narrow an audience. Otherwise all those it might potentially help, but who currently wouldn't feel comfortable here, are going to feel left out. And that's a feeling nobody wants to encourage in someone who is feeling down and out as it is.
Worth a visit, and perhaps a revisit sometime down the line, and worth using if you have the friendly support you will need, already. If you don't, there are other places to seek help with depressive illness so don't give up looking for people who will understand.