I wouldn't want to comment on whether it is a masterpiece or not: that's for time to decide, to be honest. But I think it is the best of the series so far. Despite being bigger it is less rambling than the last book and it is interesting to see that the characters of Harry & friends are much less pleasant than in earlier books - constant bickering and loss of temper: just like teenagers, in fact. It lacks the originality of the first book, but that is inevitable since the wizarding world is now constrained to be like the earlier books, whereas the first book was 'all new'.

All in all, a very successful sequel.