I first saw this movie with almost no expectations. Now I own it and have watched it more times than I can remember.
I have heard that this movie caught a lot of flak about not being "realistic" about mental illness, and not providing a diagnosis for Joon, etc. I think that it is more realistic for the "average" person with a mental illness than any other movie I have seen on the topic. There was no theme of "being institutionalised forever" and there was no unrealistic expectation of a "cure".
The character of Sam put it best when he said "Aside from being a little mentally ill, she's pretty normal." I think that's the best quote I've heard in a movie, on that particular topic.
I also think there is a reason for there being no stated diagnosis of Joon. She personifies those of us who can't get on with the things we want to do in life because of a mental illness and treatment getting in the way. She does it very well, down to the mannerisms. If she were to be labeled, say, obsessive-compulsive, or post-traumatic stress disorder, or schizophrenic, or bipolar, then the universality would be taken out of it and it would suddenly only apply to people with one certain label.
This movie did wonders for my family. Upon watching it, we all said "That's us!" and learned to laugh at ourselves and the situations we got into. It offered me a lot of hope -- what more could I want than to find an understanding and eccentric friend to love and move into my own apartment, away from the mess of hospitals and doctors? It still makes me laugh every time I see it, and "Joon" has become a household word... I recommend this to anyone, particularly anyone with a mental illness, and their families -- it might lighten things up, but it certainly doesn't skip over the bad parts.