I put an 8. It's a difficult thing to explain, the situation that I've been in for 6-8 years now. I'm not necessarily 'sad', I just don't find enjoyment from anything, I can't pursue aims, goals etc almost all of the time, and I also don't feel sadness, happiness etc. It's like an emptiness, and that is sometimes hard to deal with, because it makes all objectives in life seem pointless, when you're unable to feel the satisfaction or disappointment from your efforts. I've had suicidal thoughts (quite serious thoughts) over the last few years, and over the last few months especially, although I'm in a fairly stable place right now.
I wouldn't put a 9 or a 10, because I'm not "in pain" or "suffering", it's just a constant emptiness, and that makes things hard to appreciate.