I'm currently doing A level French and am easily making my way through the Harry Potter series in French, loving it as I go. Having not read the English version for about 10 years, I'm enjoying this for the story as much as anything else! Would recommend that the best way to read it (regardless of the language you buy it in) is to not read it alongside the English version - I am seeing it as a book in its own right; and that way you don't get caught up in whether it's translated perfectly or not. Excellent for your grammar mainly; learning nuances that you would not otherwise come across in French lessons is obviously going to take place when reading any text. People sometimes complain these books are in the past historic, however that is obvious - it's a tense used when writing novels etc. and it is extremely easy to recognise the verb each conjugation comes from. If nothing else, it helps to build up a natural command of French; applying everything you learn in lessons to something a lot more rounded and concrete + I certainly feel that the smaller things of the language, e.g. 'do I used d'autres or des autres here?' are coming without thinking because of having read so much. Very good idea to consolidate this with watching the films in French too ;) (Mainly so you remember how to pronounce Hermione the French way!)
Personally I prefer the older covers of these books (borrowed the Philosopher's Stone from my college library to read in French first and that was a 2007 version so slightly disappointing to have to enter into the new collection) but it's still also a nice book altogether.