What a disappointment.
I was really looking forward to finally seeing it.
It caught me off guard on several sides.
The whole movie felt like seeing a 3 year old in the body of an adult: always asking banal questions, repetitive and insistent.
Hoffman nevrotic acting feels way overemphasized, as if an embryonic and raw shadow of some of his great later works.
His inner parable feels scattered, incoherent but not as that of someone finding his own way or fighting society's imposed role over him or struggles, just a sloppish imitation of what it might be representing it: he passes from the indolence of the party to overexcitement and clumsiness in the hotel, to frustration, to despair.
And while I clearly appreciate a wide range of expressions and feelings from an actor/actress, they have to match some sort of interior journey depicted, having a meaning – unless the character has some motive to act in so many different ways.
Even Simon and Garfunkel's wonderful and melodic soundtrack doesn't seem to match the story's development, mood of the scenes nor character's spiritual journey or state in that moment.

The main question though is - why? Why do many of the characters make those choices or actions? There doesn't feel to be any coherence nor explanation (expressed in words or actions).
[spoiler] Why does Mrs. Robinson decides to seduce Ben? You're immediately thrown in her attempt 2 minutes in, and at first it seems like that of a bored person trying to shake things up. And it goes this way for good part of their affair, with the idea thrown in of Ben trying to get more out of it as he asks questions in the bed. That also feels forced as he presents it as a pedantic kid asking "but why" to any of the follow up explanations to the original question, rather than a vulnerable man questioning the current relationship he's in. Here we get the whole explanation from Mrs. Robinson on her actions, which indeed goes into what most viewers might have thought: an unhappy marriage, the spouses sleeping in separate bedrooms. And here we see some fragility from both late in the sequence, in one of the best passages of the movie. We also see her disillusionment over herself and Braddock as well, which turns to jealousy as she sees him starting to date Elaine - all in 5 minutes time. And Benjamin? Seems like Tobey Maguire's bad-guy-Spider-Man: forced, out of character, bad written. Sunglasses, he takes Elaine no a strip club for their date and as she cries and runs out he kisses her and voilà, she gets back on board. Couldn't it be shown in a more credible sequence?
Then, second date the following day which brings to the reveal of the affair and consequent flight of husband and daughter.
We can then forecast Ben's attempt to win Elaine's back: it indeed happens, as he tells his parents they are getting married. Before having solved the situation, before having talked to her, not even depicted as a wishful thinking, hurt man trying to convince himself he'll get her: no, just as a joke thrown there.
He then follows her around and when he reaches her on the bus, she acts a little resented but nothing more as they get to the zoo. She then decides to go to his room (why then and not on the bus or zoo?) only to get a 10 words explanation on the affair, scream, get calm and smile at the landlord when he comes, sitting on the bed sipping water, passing then to forgive Ben as he packs up with no apparent inner transaction of the character.
Ben keeps then insisting on his marriage proposal in a sitcom-suspense-of-belief series of scenes, with Elaine pretending like everything's fine and even the classic "screaming in the library" one.
"Are we getting married tomorrow?"
"No"
"The day after tomorrow?"
sums up the whole sequence.
And while Elaine is comprehensibly uncertain on what to do, she still passes from refusing his insistence to kissing him after the library scene, with no apparent constancy of sentiments.
Husband's scene, then.
No introduction, tense silence, tiredeness or any of the feelings you might imagine in a man in such a situation:
"Do you want to tell me why.. you did it?". Like that, out of the blue.
Short dialogue,
"... we are getting divorced"
"But why??" Ben asks. Why, would YOU ask, he asks such a question.
"..We might just as well have been shaking hands" he proceeds to explain his several-nights-long-affair to the husband.
After the exchange finishes and Mr. Robinson storms off shouting, Ben decides to ask for some change to the baffled landlord who overheard the final insults on the stairs. Way to underline the gravity, intensity of the moment and the husband's shuttered feelings.
Elaine then leaves school out of the blue, says she loves Ben but it would never work out. All this after having passed from hating him, to forgiveness, to almost accepting his proposals. Abandoning her whole life and career prospects.
Ben then breaches the Robinson's property and has the first exchange with Mrs. Robinson after the big reveal of the affair. He then blames her for Elaine's lack of cooperation in seeing him (!) and in 30 seconds he's gone as the police arrives. No confrontation, no clash or reconciliation or whatever you may have expected, if you expected some exchange of some kind to happen between them in such an intense reencounter.
We then hear for the first time the iconic “Mrs. Robinson”, which gets interrupted for Ben’s inquiries at Carl’s frat, only to resume shortly after – again not really matching the mood nor scene.
Some more rushed inquiries.
Remember, all of this happened suddenly – with no build up.
Ben crashes Elaine’s wedding with poor Carl, then with an improbable screaming and pounding on the glass next to the pipe organ, he manages to get the bride’s attention as she has just kissed the groom… and just like that, by screaming “Elaine”, after her note, the rollercoster of unexplained forgiveness and redention, some close-ups of the Robinsons and the groom not acting in any way but grinding their teeth at the camera, Ben fighting – in order: the father of the bride and a whole bunch of attendees at the same time, fending them off with a cross - … they finally run away in a municipal bus.
The end. [/spoiler]

It’s complicated to judge a movie almost 55 years later, but the elements of a story well developed remain ageless in my opinion, and I just couldn’t find them here even remotely.