I think this movie is good because it's real. There's unpleasantness, there's disgusting behavior, and there's some truly questionable actions taken by the father, (Robin Williams.) This movie isn't for everyone. If you aren't comfortable with heavy sexual behavior, or the topic of suicide, this might not be for you. However, I highly recomend you give this movie a try.
I find this movie enjoyable because it doesn't give you a hero. There's nobody to root for, because everybody is messed up. It doesn't necessarily make the movie satisfying, but I think that's because we enjoy projecting onto the "hero," the "perfect" person, but when there's nobody to root for, it makes you feel guilty as well.
I don't know. I just found this movie to be really emotional, and I think it is a great movie if you're looking to feel something other than satisfaction and joy.
Give it a shot!