Only watched episode one, so my opinion may change...
The scenes in Sheffield with the younger versions of the main characters are wonderful, with great acring from both the lead actors and the ensemble. Well chosen locations and an evocative soundtrack really help to put us in the time and place, and create a sense of those teenage years in Sheffield.
The older versions are less successful, especially the Australian scenes, and the actor playing the older Alison bears no physical resemblance to the younger one, and her personality seems utterly different. Poor casting.