Actually I'd give it 2.5 stars. It's better than the first book in the series, but I found it not as good as the second.
The first half of the book was a bit boring, but the second half finally got interesting.
I liked the adventures they had, they were really creative, especially the part about the stars, but I still found the plot quite poor. They seemed to be just wandering from island to island. In the previous books, they had a mission to accomplish themselves at Narnia and were taken there because of that; in this one, however, they just happened to come across Caspian's ship and join his mission.
Eustace wasn't the most likeable boy, but I had a lot of issues with the bashing he received from Lewis and the latter's comments. Even knowing who I was reading, the way he presented Eustace's up-to-date family made me want to get in the TARDIS and go back in time just to punch him on the face. OH MY GOD, they were vegetarians! And had little furniture! And the windows were OPEN! Yes, open - what a horrible family! And the worst of all: Eustace's school didn't have corporal punishment. What kind of education is that? Where is this world going to? Yes, his parents were the worst. Oh, and I almost forgot: they apparently taught their child to treat girls a bit more equally. That's the devil's work, for sure. Thanks Aslan good ol' gentleman Caspian was there.
Nevertheless, with all the sexism these books contain, I still think Lucy is a great and strong character. I like her.
Speaking of the Pevensies, the explicit differentiation between Peter's and Susan's current locations was terribly -and sexistly- annoying.