Pros
I have been working as a part-time employee at the Co-op for a few months. I work one night and one late evening per week on a 12 hour contract.
If it weren't for the Co-op selling off many petrol stations, I would continue to work there. The work is steady, reliable and easy. I know I can't speak for every Co-op, but my colleagues are friendly and often work can feel like more of a social event than a chore.
The managers are very reasonable. They want the work done to a good standard, but as long as that is fulfilled they won't nit-pick about how it's done (provided policies are followed). My store manager is also very reasonable, and he puts genuine effort into ensuring that everyone is doing well.
The pay is fair for the work, and the benefits are helpful. Their free healthcare benefit is probably my favourite, being able to call up a doctor without going through the NHS can save a lot of time with less serious issues.
Cons
I haven't worked in enough retail places to point out cons about the Co-op.
