I've read the German and the English books. The translation was very good for me when I was 9 or 10 years old and started reading HP
Well, no, I still buy the German books because I don't want to miss a single detail. The translation is quite good, what's to criticise is that there are a lot of, uhm, regional words. For example, "right old slob" (said by Tonks in OotP) is translated as "Pottsau", which is a (really ridiculous, he he) word only known in the north of Germany, but not in Bavaria, Austria or Switzerland.
The translator tried to imitate JKR's style and it didn't really work. Sometimes there are very exaggerated, old expressions and then again it seems pseudo-cool and slangy. Especially Ron's way too "offhand". The translation is not as bad as it now seems, but not a quarter as good as the original edition.
However, I like the translation of the names. Many names are not translated, only a few like the "Grubbly-Plank woman", who became "Raue-Pritsche" (pretty much the same meaning) and Hermione (Hermine). Hermione's translated because the pronunciation is very difficult - the meaning changes, but, so what, I for my part like Hermine.