My stance on masonry arriving in CSS is pretty clear: get the focus situation sorted and then I might be more interested. I personally think masonry is quite an antiquated design pattern, but it still finds itself on the web a lot so not needing JavaScript for layout is only a good thing.
I’ve not been subscribed to the idea of masonry being part of grid at all really. I was hovering more on flex because masonry — to me — is a flexible layout which packs itself based on the content with some light column sizing rules. I like the idea of display: masonry more though because like Rachel says, you inherit all sorts of defaults from grid and flexbox. Having a block-like specific value for display feels like the most sensible idea to me.
Consider this my upvote for display: masonry.