Too misleading a movie - deserves the flak
This was marketed as a comedy. It isn't a comedy. Pretty much nothing in it was funny. It was nothing more than a mother-son relationship drama set in a car. No, I'm not expecting a "bromedy" like Road Trip or a Harold & Kumar style wild ride, but I was expecting at least a decent amount of humour in the dialogue like in other Seth Rogen movies (e.g. Knocked Up, Observe & Report, etc), which we had none of because Seth's character was just utterly depressed throughout the movie.
I mean, look at the official trailer ffs:
https://www.youtube.com/watch?v=7FMQLzOq1i4
The film didn't even live up to the title name - where was the "guilt" part of the "guilt trip"? There was no guilt at all in the movie, only regret. It should have been called "regret trip".
And regarding the "guilt", I get the impression that the mother was meant to have been more annoying than she actually was. Those who watch the Big Bang Theory will get this, but I was expecting this movie to have the equivalent of Howard Wallowitz's mother being thrown into a car with Seth Rogan. Instead all we had was a loving mother with a tiny amount of nagging about drinking water and past relationships - the mother wasn't anywhere near as entertaining as I had hoped. Hence, the son just came across as an utter douchebag because of his attitude to said loving mother (until he changed and mellowed out after getting punched in the face), which made the movie very awkward.
Overall, it's not a terrible movie in itself, but I do think the low rating is justified and reflects the misleading nature of the way this film was promoted and advertised.