The Spanish translation is very good and keeps really closely to the original. The joke is awesome across well, just like in the English edition. (Except for some language-related jokes, their implementation is problematic in any translation into another language. But this is, has been, in my opinion, an elegant solution.) Another (smaller) plus the nice (but "childish") management of Covers.
The language is very easy, because a lot of direct speech occurs. After I had a semester Spanish at the university, I was able to read the book without any major problems. Of course it was an advantage that I already knew the story, but the learning curve when reading was still surprisingly large and my ambition was learning alongside more inspired!
I recommend this book to anyone who wants to brush up their Spanish or just have so interested in Harry Potter and the language and would like to read good books in Spanish. I recommend this book as well as beginners in Spanish, who already had a year of training. But if you already know Harry Potter, you can calm rather venture to the Spanish edition also. You will not be disappointed! In any case, you will quite incidentally to improve his Spanish. ^^