The ISM sets the tone of the service.

I had a CX flight in F, and believe it or not, as both OWE and F passenger, the ISM did not even bother to greet me throughout the 12-hr flight, and in the galley, pretended that I did not exist in numerous occasions when I was going to use the toilet.

And that flight, I had one of the worst service ever, despite being in CX F. (Only the F cabin purser was ok, the senior flight stewardess in F cabin gave service that is of Y cabin. Hurried, showing unhappiness at every request, and I got that from her from my very first interaction, as if 1 request is simply 1 too many - she sighed and shrugged her shoulders when I said I may want to have my meal service later.

Towards the end of the flight, around 45 minutes before expected landing and even before captain announced "cabin crew, 30 mins to landing", she wanted to clear my cup of water, and when I said I am still drinking, she remarked "Please understand I have a lot of things to do" Wow.
And ISM is non-existent, so you get the point. )

And when the ISM goes round greeting, I can see a marked difference in overall service as well. I guess this is leading by example.

Think I should find time to write in and give feedback on that ISM. Believe there are more worthy pursers that should be given the chance in that role, if that ISM is not interested in her job.