I have spent the past two months working at three different farms in Kyushu, each of which gave me a very different experience.
I first went to host 047 in Oita. This is the place to go if you really want to be a part of the family that you stay with. The work is not too hard and you spend a lot of time talking with the grandmother and three kids. Also, if you are planning on wwoofing in summer i would suggest that this is the best place in Kyushu as it is surrounded by mountains and there is always a nice breeze. The host father also said to me that he would like more wwoofers in the future... up until now they have been few and far between.
Host 097, also in Oita, was the only one of my three hosts to take the Organic part of WWOOF seriously. You should definitely investigate going to this host if you are serious about farming, the work is not that hard but if you want, you will be able to learn more than you ever imagined about Organic Farming (in Japanese or sometimes with the help of a dictionary, in English).
Finally, I went to host 176 in Nagasaki ken. I was the first WWOOFer this host had so intially, they were very worried about everything. However, they soon got used to the presence of a foreigner and i had a wonderful time there. I would recommend that you try and stay for a longer period than I did as they really want to get to know WWOOFers well and will take you out to many different places after the work is done (in one week alone, i played volleyball 3 times and went to 2 parties).
I would recommend all these hosts as places where you can really have a lot of fun and be a part of the family, not just during working hours.
(male, from Australia)