I have experienced it once in Japan. It was an interesting experience as we lived with a family and their three young kids. We did lots of bean sorting and picked beans out of scrap plants but it wasn't overly exciting. My boyfriend had a horrible experience as the the house caused him to have an asthma attack that medicine wouldn't help. We did get to try lots of Japanese home cooked meals that I have not tried anywhere else so that's one great benefit and you do get to really experience what it is like to live in a country household. This was one experience with one host and I have had better experiences wwoofing in other countries. Many people have amazing experiences, it's important to just make sure you know everything involved and what it will be like before you get there. You can also try helpx or workaway as they have similar placements. The cost is minor really, just the cost to get there and the membership fee. Plus you can always leave if it's not working out. I would suggest picking somewhere that is in an area of Japan you want to be in anyway.