This movie definitely depicts an outlandish circumstance that would never occur in the real world, no woman is going to sleep with a man who has also slept with her mother AND her grandmother, especially after briefly suspecting him to be her father! But, if you can overcome the "ick" factor of that particular dynamic (it's based off of the Graduate which serves more as a premise, rather than the plot, so not entirely impossible to do) it's still an entertaining ride. It kept my attention, I stayed engaged up to the end.
Though, to be more specific, I think a lot of this movie's appeal comes from it's cast. There's a really good dynamic and they play really well off one another. Kathy Bates (though her time is short lived) is wonderful, Shirley McClain is fun and hilariously sassy, Kevin Costner is incredibly charming and more handsome and than ever, and Mark Ruffalo is just so endearing, you wish it were you he was proposing to.
I mean, is it an amazing, unforgettable piece of cinema? Is it worthy of tons of awards and accolades? Of course not. But it's a fun "Sunday afternoon cuddled on the couch in your pajamas" type of movie and I enjoyed it.