The Kingdom of Sweets was a roller coaster -- it started off fantastic, then I was a little less interested, and then something unexpected happened and I was all in again... and then I got bored.  And unfortunately I stayed relatively bored for the remainder of the story. 
I think there was too much trying to happen and it didn't fit in enough with the Nutcracker story. 

I didn't particularly like any of the characters.  Both Clara and Natasha were rather unbearable.  My favorite was probably Drosselmeyer

and even then I liked him more when he was a puppet

 

To be completely honest, I didn't understand most of the story. I mean, the big plot points sure, but what was the motivation? And what was going on at the end?

I don't understand why the Sugar Plum Fairy was so evil, or even what she was, cause she wasn't actually the SPF but some sort of dark  shadow figure. 
What was her deal with Drosselmeyer? If that was mentioned then I missed it, but I don't know why she wanted him and why he was scared of her or even how he evaded her all this time. I think we were told it had something to do with that mirror, but if so, how? 
I'm also left wondering why the bit about the Russian Revolution was added. The connections with the uprising poor were loose at best, and the ending with the Lenin reveal felt unnecessary. I think this book would have benefited from remaining a fantasy alone. It started to border on a historical fiction and it didn't work for me

I just really had too many unanswered questions by the end for me to enjoy it.