Rowling is an imaginative writer who has the ability to create new, exciting worlds. Is she a good/great/best writer? No. Her use of prose and description is poor at times ('Snape said SILKILY), and her vocab' leaves a lot to be desired ('his only chance was to keep hold of Quirrell, keep him in enough pain to stop him DOING a curse.')
Rowling - as deserved - has many fans who adore the characters, worlds and story she has created. This sometimes, however, makes people blind to her faults as a writer.