I actually love most of this book, in which Our Hero Harry is entered into a dangerous international magical contest. I love that Rowling's world is given some space to breathe here - it's not as tightly constructed as the first three, but that's made up for in part by the inventiveness of the worldbuilding.
I do have a couple of reservations, though. First, there's SPEW, the society Hermione sets up to save house-elves from slavery. Everyone else treats it as a joke ("the house-elves like working for no pay!") and I'm not sure where Rowling sits on the issue; certainly nothing comes of it by the end.
Secondly, the ending, which is where it all falls apart. Harry's final confrontation with Voldemort is frankly and unfortunately snoozeworthy, and Voldemort's reasons for putting in place his extremely elaborate and laborious plan are very sketchy indeed. (He doesn't actually need Harry, and revenge may be a dish best served cold but a whole year is just really fucking stupid.)
So, yeah. Enjoyed it until the last, oh, fifty pages.