Pros
Really nice people; great working environment; good work life balance (typical day 7.30am - 5.30pm). You can move up the ladder although it will often take quite a few years. In summary - If you're the kind of person who wants a good steady career for the long term, M&S could be for you. If you want to deliver things, progress and learn fast, you will find it a frustrating place to work.
Cons
The Head Office is like working in a 1970's government department. It is very hierarchical, bureaucratic and slow moving. Decisions are often made based on multiple hushed conversations in the corridors rather than deep analysis and data. It's a reactionary culture. Lots of firefighting of problems. People are praised for working late to recover an issue, rather than actually fixing the issue to make sure it doesn't happen again. Quite a few problems are recurring. It is very difficult to deliver change. Doing so requires the agreement of so many senior staff from multiple departments who often have conflicting objectives and agendas.