Yes. Both.
Sometimes I love flashbacks in fiction. And sometimes I loathe them.
I love flashbacks when they are done well—when there is a scene from the character’s history that explains why they’re making the (often stupid) decisions they’re making today. Or a scene that explains the predicament they’re in today, and how they got there.
One novel that does a great job of using flashbacks is Out of the Cages by Penny Jaye. The present story is that of a Nepalese teenager who has just been rescued from sex slavery, and her battle to find a new normal life. The past story is how she got tricked into prostitution in the first place. It’s a tough read, and flashbacks reinforce the current story (click here to read my review).
More often, I loathe flashbacks. Why?
Because the flashback isn’t sharing vital information. Instead, it brings the story to a halt while it takes us back into the past to pass on information that’s only vaguely relevant to the plot at hand.
The worst example was a book I read years ago, where the author kept interrupting the hero and heroine’s story to take us back to how the heroine’s parents met and married … and to how her grandparents met and married. It wasn’t that the writing was bad. It was that I didn’t care—I cared about the hero and heroine, and they were the characters I wanted to read about.