Let me just start by declaring my belief that JRR Tolkien's "The Lord of the Rings" is one of the greatest works of fiction -- not just fantasy, but ALL fiction -- ever. And I've read a lot of fiction in my life! Still, the main question here is whether or not the Spanish language translation does the original English book justice. Unfortunately, I'd have to say that from that perspective, "El Senor de los Anillos" has some serious problems.
The main problem is probably inherent to ANY translation of Tolkien from English into another language. Professor Tolkien, as a great student of language, chose his words, accents, and dialects extremely carefully. Unfortunately, many of these choices, such as Sam's working class speech patterns ("coneys," "Gaffer," and all his many idiosyncratic sayings and references), or the Orcs rough (Cockney?) accents, just don't really come across in Spanish. I have found the same thing in other translations, such as when I read "Tom Sawyer" in French back in High School. Same deal.
In addition, however, I also caught numerous instances of outright errors in "El Senor de los Anillos," which in the case of Tolkien and his care for such things, are truly egregious. For instance, on numerous occasions the translator said "East" when he meant "West" or vice versa. Not good, especially when these directions are fraught with meaning in Tolkien's conception of Middle Earth. Less egregiously, I found some of the literal translations -- "Bolson" for "Baggins" or "Comarca" for "Shire" simply to be annoying. Why translate proper names? Must be a translator thing; I just don't understand!
Finally, a problem both with translation into other languages as well as into film is the loss of much of Tolkien's poetry. In the case of the blockbuster movie trilogy, poetry was kept to a minimum in favor of a predictable (heavy) emphasis on action, action, and more ACTION! In the Spanish translation, the problem is that the rhyming schemes which Tolkien uses, which are critical (in my humble opinion) to a true appreciation and enjoyment of the poems, are basically demolished. Maybe it wouldn't matter so much with other authors, but again, in the case of Tolkien, where each word choice is given tremendous thought and care, alteration of rhyming patterns can only lessen the impact of what Tolkien is getting at and the sheer beauty of his poetry.
Having said all this, despite the frustrations mentioned above, I enjoyed reading Lord of the Rings in Spanish, which I mainly undertook as part of my studies of the language. Reading a familiar book in another language is not a bad way to improve your vocabulary and general command of that language. Still, I would love to see a translation of "Lord of the Rings" that comes without the sloppy errors and that rings truer in spirit to the original.