Definitely worth seeing in the theater. I don't think I would have reacted the same just watching at home.
First half or 2/3 was just totally depressing, to the point of being uncomfortable.
The last part of the movie was pretty crazy and disturbing as has been mentioned already.
Phoenix was excellent and should get consideration for an Oscar.
Even though the film was really really good I didn't walk out of the theater thinking I can't wait to see it again.