There are a few problems with her claims, chiefest of which is that correlation is not causation. In particular, unless the studies and effects are large, her sample sizes would have to be huge to account for "grit" being a major factor. A cursory search of the literature indicates that, for instance in her study of which cadets make it through the Beast Barracks:
what happened is that 95 percent of all cadets make it through Beast Barracks, while 98 percent of the very "grittiest" candidates made it through.That's just 3%. Further down the NPR article we discover that the correlation of success with Grit is 0.18, while the correlation between SAT scores and performance in college is 0.5! In other words, given the choice between being gritty and being smart, you should definitely choose to be smart!
It would be one thing if Duckworth acknowledge all these flaws in her book, but chapter after chapter of the book is about how important Grit is, while sweeping aside any issues. She even discusses how to train grit into children.
To her credit, she does acknowledge that you can't just have someone tell you "get good at piano and improve your grittiness." One of her points is that no one can impose your goals on you --- you should be the one choosing your instrument, or the task or skill you would like to improve. Any other approach (including the typical tiger mom approach) is likely to fail.
And ultimately, grittiness can backfire. In John T. Reed's book, Succeeding, he discusses his teenage goal of getting into West Point and graduating from it. He succeeds in doing so, only to discover that military life wasn't actually very good for an intelligent, driven person, and that his personality was far less suited for it than he had imagined. In that book, Reed points out that picking your environment to suit your personality and strengths is actually far more important than the other attributes that others allude to.
In short, I don't think Grit's a worth while book, especially given its flaws and the author's unwillingness to point out the flaws in her research. Not recommended.