At Slate, Dana Stevens and Culture Editor John Swartzberg (I'm not sure of the spelling) have an audio review (14:57) of the 'mutual degradation' they suffered watching 'I'm Still Here':
The worst thing about I'm Still Here (Magnolia Pictures) is the fact that it exists.
Articulate, well-versed, and totally damning. As for the question of whether the project is real (and Joaquin Phoenix is going down in flames) or an art experiment (and JP, and to a lesser extent Casey Affleck, is an acting/directing genius), they split. It's good listening.
On the general quality and watchability of the experiment/documentary -- 'Feels incredibly long'.