It worked for me. On both my iPhone 15 Pro Max, not on my Intel iMac Pro.
It works for the "article" (ie
https://forums.macrumors.com/threads/ios-18-how-to-use-safaris-new-highlights-feature.2436719/ ) but not for the MacRumors front page (which I guess has too much going on to be "summarized").
A more interesting case is something like
https://www.matthewball.co/all/marveliadpart2
This is a mildly text-dense page, and presumably the sort of thing that some busy readers would like summarized. And yet no sparkle. Like this article says, right now the feature seems more like it's for extracting factoids than much else.
Seems like a help for people who are not good at skimming a page fast to extract what they want, but not much use to people who already have that skill?
As someone pointed out, the name is Highlights, not Summary, and it seems like a good idea to keep that in mind. Summary hasn't yet arrived.
But there are a bunch of weird aspects (SW a little rushed)?
(a) I get the sparkle when I refresh this page. But the sparkle disappears once I scroll around, even if I scroll back to the top
(b) When I refresh the page there's a brief flash in the URL bar saying "Summary and Reader available". Point is, parts of Apple are calling this feature Summary, others are calling it Highlights! As I said, Highlights makes a lot more sense...