No substitute for the original / no es buen sustituto por el original
Customer Rating: 
Look, this book is a classic in its original English version. Though the Spanish translation is not deficient, it just does not have the same charm that the rhymes have in the original.
Get this book in English, and choose books translated into Spanish whose text are not central to their enjoyment (e.g., Buenas noches, Gorila), or choose books originally written by Spanish language authors.
Este libro es un clásico en su idioma original: el inglés. Aunque la traducción al español sea excelente, no está a la altura del lenguaje de las rimas en inglés.
Aconsejo que compres este libro en inglés, y que compres libros traducidos al español que no dependan tanto del texto (e.g., Buenas noches, Gorila); o, compre libros escritos por hispanohablantes.
Does not lend itself to Spanish
Customer Rating: I was so excited to find Goodnight Moon in Spanish, as the English version was a favorite of my siblings 20 years ago, and my 15 month old son today. We have dozens of Spanish translations of English books, some of which work better than others. I had hoped this one would at least be okay, but for me, it seems very awkward and loses the charm of the original; and my son will not listen to even a page read in Spanish.
Others have commented that the book keeps the rhythm of the Spanish language, but I couldn't disagree more. I am hesitant to simply blame the translator--with such simple images, I think there is only so much a translator can do to preserve the feeling of the original, and perhaps this book just didn't lend itself. All the charming rhymes seem to be lost, without being replaced, and so the repetition that is so soothing, even to babies, just isn't there. In addition, the lines themselves feel so much longer in Spanish, as well, and my son, who adores the English, gets very impatient for the page to be turned.
Let me just provide a couple examples. From the first page to the second:
English: In the great green ROOM, there was a telephone, a red balLOON, and a picture of...the cow jumping over the MOON.
Okay, maybe "room" is a stretch there, but there is a rhythm to that rhyme.
Spanish: En la gran habitacion verde, hay un telefono, un globo rojo y un cuadro...de una vaquita que salta sobre la Luna.
Some other pages are slightly better:
English: 2 little kittens and a pair of mittens
Spanish: Dos gatitos juguetones, dos caletines y dos mitones
But followed by:
English: ...comb and a BRUSH and a bowl full of MUSH, and a quiet old lady who was whispering HUSH
Spanish: ...cepillo y un gran peine y papilla en un tazon, y una amable viejecita que teje muy calladita.
So is there some rhythm in the Spanish translation? Sure. But does it even approximate the English? No, I don't think so. Of course translations are not going to be perfect, but we own many Spanish texts [translated from English] that are virtually as good as (or even occasionally better than) the original. The awkwardness of this translation seems to present a bad example of the beauty and flow of Spanish, and unless reading the English version is not a possibility, I really don't recommend this book in Spanish--especially not with a baby or toddler. An older, school age child who wants to learn the Spanish vocabulary would, I think, be much more likely to enjoy this book (thus the 2 stars instead of 1).
Note for reference: Some board books we have enjoyed in Spanish include "Donde esta el ombliguito" by Karen Katz, and all the "Este no es mi..." books (Este no es mi dragon being a favorite among favorites, though, to be fair, some of the word choices are a bit odd, but that doesn't affect the overall flow of the book).
