When I finished reading Defiant, my first thought was that it was the second best full novel in the series, and this was held up when I checked my intended rating against the other three novels as it rated higher than both Starsight and Cytonic, but not as high as Skyward (if you're interested, I rated three of the novellas the same as Defiant, while Evershore is up there with Skyward). 

Much of what I love in Skyward and the novellas is also here in Defiant: Skyward Flight, slugs, a focus on friendships and family, the theme of finding yourself.  I like that while much of the book remains first person from Spensa's perspective, we do occasionally get other characters' points of view also. 

I'm not without criticism. There was one particular point in the book where a common trope I really don't like came into play, and it took me a while to get over my frustration and get properly engaged in the story again. I also really would have liked to see more of Jorgen in the book, he felt somewhat underutilised. 

Overall I did love Defiant and it's a very decent end to the Skyward series.