I originally didn't watch this because I haven't really appreciated De Niro comedies in the past. There was also the usual bad press because this movie uses a rather typical plot formation that can be successfully guessed in the first 10 minutes of the movie with a few slight twists.
The story is pretty straightforward. Jason (Zac Efron) is a successful yuppie lawyer who is a bit of a stiff who doesn't know how to have much fun and is instead extremely dedicated to his career and fiance (nothing wrong with that, of course.) At his grandmother's funeral he is approached by his grandfather DIck (De Niro) who asks Jason to drive him to Boca Raton, Florida for some reason I don't remember.
The invitation seems innocent enough because Boca Raton is a retirement area and well De NIro is old. Jason finds out very quickly that his grandfather has kind of lied about his objective in asking him to drive to Florida as he has them drink-driving almost immediately. Dick is very good at talking to people and has him and is grandson talking with young attractive people on their way to Daytona Beach (a party beach, for those of you that don't know it.)
Grandpa gets them involved in a bunch of fun things that Jason is too prudish to participate in but ends up eventually enjoying. Grandpa intentionally drugs Jason at a party and this results in Jason finally letting loose. A bit too loose actually as he ends up stripping down, smoking crack, stealing a motorcycle, and sleeping on the beach.
anyway, the story become predictable from about the first diner encounter on the way to Florida, but that doesn't really matter. I found the movie funny despite its shortcomings and dumb plot twists.
The movie didn't get great reveiws because it follows a very typical storyline formula. No one will ever change this formula because this film made about 70 million dollars. It's a fun movie with lots of laughs. Just don't expect anything amazing or new.