Setting or mixed attainment?

My current school and the two before it have all in the last few years ditched setting in favour of mixed attainment classes. Two of them for all subjects, while one has retained sets to reflect tiered maths and science papers. School A is a leafy middle class school, school B is an average comprehensive and school C is a Requires Improvement school. At each, teachers have quietly admitted to not liking the idea for one of three reasons: because of added complexity to planning; because they believe it lets down HA students; or just due to a fundamental belief in setting.

What is your experience and which do you think works best?