The good: the book is engaging and detailed, and paints vivid pictures of Charlotte, Victoria and the people in their lives. The protagonists' faults or weaknesses are not overlooked, but help create rounded portraits. The writing is easy to read, but doesn't come across as lightweight. I would call it a good introduction to their lives, showing clearly how many hurdles they faced.
My reservations stem from the section on Victoria, because I am more familiar with her life than Charlotte's. Principally, the portrait of Albert that emerges reads more like caricature - the stodgy and conniving Coburg princeling - than the highly intelligent, passionately loving and inexperienced twenty-year-old he was when they married. His humour (which wasn't on display for strangers; he was shy) never rates a mention, and the author seems to take issue with him wanting to do more than just be a decorative royal stud. The overall tone leaves one wondering if Albert loved Victoria at all or was just manipulating her - an entirely unreasonable stance, given all the evidence of their lives.
Another claim made in passing has, in my opinion, no place in a serious biography. Saying that "surely" Stockmar arranged for Albert to visit a courtesan before his marriage smears both men as hypocrites. Albert's whole family background, with his mother divorced by his hypocritical and promiscuous father, and his brother's acquisition of VD at an early age, clearly made sexual license abhorrent to him. There are simply no grounds for supposing he would have visited a prostitute in preparation for his marriage.
There are also factual errors. One simple one should have been caught by editing: calling Melbourne head of the Tory party. Another shows missing research: sticking to the old tale that the dress Victoria wore to her first Privy Council was an old dyed outfit. Kay Staniland's "In Royal Fashion" discusses the dress (which survives today) and makes it quite clear it was not re-dyed. Staniland's book is not new, and given that it is about the dresses of Charlotte and Victoria, I was surprised to find it wasn't listed in the bibliography.
My point in detailing these things is that there may equally be as many inaccuracies or dubious interpretations in the section on Charlotte. As I said - it's interesting and lively, a good introduction, but I wouldn't take it as Gospel!