I'll throw my two cents in.
I break sins into three catagories. From (in my opinion) least bad to worst
Sins that hurt oneself
Sins that hurt others
Sins that create more sinners
At most masturbation hurts oneself.
Having tried an earlier stated experiment of conciously going without masturbation, I noticed that without the occasional release my aggression was increasing, my self control over my temper becoming dangerously weaker. I went into a slight depression and it took everything I had not to haul off and beat people who angered me. This was most likely also due in part to other factors in my life, but when I stopped my experiment and began masturbating again I also found it much easier to control my temper. I became less angry and generally a happier person again.
So perhaps masturbation (or more accurately the lust involved) is a sin, but if it helps (which I believe) me to keep from violence against others (which I consider a worse sin) then I will continue to do so.
What I find really interesting is the occasional person who seems to think masturbating is worse than fornification and use that as a subtitute. In old law you could be killed for fornification, but I'm unaware of there actually being a punishment for masturbating. Can anyone explain this point of view to me?