I had the chance to see this at the Toronto International Film Festival in early September and came away disappointed.
The movie is founded on a great premise and I was eager to see where it went. Playing a brooding, lonely old man obsessed with a past relationship seems like a good fit for the veteran Pacino. A sole proprietor key cutter was the perfect choice of profession for such a character. Giving Pacino full screen time was a good choice, as I can't recall a scene that didn't feature Pacino.
However, inconsistency in the Manglehorn character seems to overwhelm a good acting performance. Manglehorn seems at times senile or rude, and at other times the clever, likable character with good advice. It doesn't help that a good explanation for his obsession with a past relationship is lacking.
There are some solid random scenes and conversations, including Manglehorn's discussion with a child or the entertaining Korine telling stories of little league. These worthwhile scenes are surrounded by just as many scenes that fall flat and make you lose interest. Too often scenes are present as filler or make you feel like a better climax is due. Manglehorn's interactions with his son, played well by Chris Messina, feel like they belong in a different story arc.
I believe that Green has the ability to make a great, subtle film if it all comes together. In Manglehorn, the script Pacino is given and the characters he is surrounded with take him no where in particular, which is fine if you are entertained or enlightened throughout the film's duration. That's not the case here. Though the foundation was there, the payoff isn't worth the attention paid.