I am not a CSM but I don't think it was the CSMs that were moaning about pay, it was main crew. In fact recently M/F have had a pay rise for the main crew and an across the board increase in their hourly rate and I think they are happier than some of these earlier posts portray.
I think the pay is very similar to Virgin. I doubt you would be able to enter Virgin as a direct entry senior crew member and therefore Mixed Fleet CSM is a much better salary than Virgin.
Mixed Fleet is in it's infancy and although the crew earn less that Worldwide Fleets and Eurofleet, BA has said it will ensure that the pay is market rate plus 10%. There have been a few problems with the rostering but these are being addressed and are mostly due to it being a new fleet and not up to size yet.
The fleet is new and therefore some of the crew are not as experienced as Worldwide or Eurofleet but they will catch up, I am sure in time.
When I see my Mixed Fleet colleagues, the majority look very happy!