The Wikipedia entry on Mrs Greville's reputation is hilariously snarky:
"Following Greville's death, Queen Elizabeth, who inherited the bulk of Greville's huge collection of jewellery, described her as "so shrewd, so kind and so amusingly unkind, so sharp, such fun, so naughty; altogether a real person, a character, utterly Mrs Ronald Greville".
By contrast, Sir Cecil Beaton described her as "a galumphing, greedy, snobbish old toad who watered at her chops at the sight of royalty ... and did nothing for anybody except the rich".
Rudyard Kipling is thought to have called her a slug.
James Lees-Milne, in his diaries, commented: "Everyone is agog to hear the terms of Mrs G's will. She was a lady who loved the great because they were great, and apparently had a tongue dipped in gall. I remember old Lady Leslie exclaiming, 'Maggie Greville! I would sooner have an open sewer in my drawing room!'"