I have read several C.S. Lewis' books in English, and enjoyed each and everyone of them, but I have never read "The Last Battle" in English.
A few months ago, I started to learn Spanish. In order to improve my Spanish reading skills, I started to read The Narnia series in Spanish. I am so glad about the choice, because the Spanish version of this series is as exciting and enjoyable to read as the English ones. How do I know? Well, at the beginning, I had to refer back to the English version to fully understand the Spanish version, but after 1-2 books, I was able to read the Spanish version without going back to the English one. Since this series is translated by the same person, I got used to the style of writing and the word choices after reading 1-2 books, and that made the rest of the reading easier. In about two months time frame, I was able to recognize about 3000-4000 Spanish words by reading 4 books from this series. Since the story is so engaging, I can hardly stop reading.
I have also asked my Spanish teacher, who is a native Spanish speaker and a professional translator, about the quality of the translation. He said the translator did a great justice to the book. He himself is a C.S. Lewis fan as well, so I trust his opinion.
As the other reviewer also pointed out, this series in Spanish is fantastic for learning Spanish. Besides learning vocabulary quickly and enjoyably, I became familiar with Spanish grammar rules after reading 4 books from this series, because C. S Lewis likes to use compound sentences a lot.