Do you think Ben had a bad experience

Yes and no...on one hand, I think it caused him a lot of trouble (complications with his relationship with Elaine, after he fell for her; the confrontation with Mr. Robinson; etc.).

On the other hand, I think it did force Ben to take responsibility for his life and gave him a better idea of what he wanted for his life. His driving to Elaine's wedding to stop her from marrying Carl...I'm not so sure he would've done that before the affair.

One final comment...at some point after the movie ends, Ben and Elaine would've had a serious conversation about their relationship and their lives going forward. That would've been some conversation for sure! 😃

I definitely would have had a good experience banging Anne Bancroft back then but to each his own.

The sex part was definitely good. The emotional turmoil and conflict were definitely bad. When he's much older, he'll mostly forget the emotional stuff, but he'll still be jerking off to memories of the sex.