Having been a reporter in Asia for many years - and in fact, she cites her works in the notes repeatedly - she certainly seems to be the right person to have written this. The main thrust of it is to inform about the conditions in North Korea, particularly during the famine of the 1990s, that led to mass starvation, struggle and desperation, and caused an increase in defections.
Naturally, it's impossible to write this with zero bias, since Demick has not lived inside North Korea, only having visited what the North Korean government deemed appropriate to visit. But I don't see that the defectors she spoke with have any reason to lie about the difficulties of living and surviving in North Korea - except perhaps to further the agenda towards reunification, in order to finally be reunited with their family and friends. And since she includes defectors who didn't actually have reasons to defect per se, the descriptions of life in North Korea are that more substantial and trustworthy.
I would recommend this book. However, it's heart-rending to read about seeing dead bodies in the street, and homeless youths stunted by food deprivation, and the continued faith in the leadership through all of this. It's fast, fascinating reading, but it's difficult to read for those reasons.