I've just finished MMA and, in the interests of making more cards and using up a smidgen of my sho-sized stack of Craft Stuff have now started listening to 'Strong Poison'. I also prefer the later novels, and I think you're absolutely right about the reason - the author knows her characters so well by then that they really seem to live.

The classism and utter dismissal of any woman who isn't an intellectual is a bit saddening (much the same with the Heyer detective novels, which otherwise have their charms), but it is interesting to inhabit a world we don't know. I think that's why MMA is my favourite—I used to work close to the advertising world, although not in an agency, and reading about a 'period' agency fascinates me.