Objectively, a mix. The plot engineering is expert, with clues fairly doled out and well-disguised and the remorseless deductive logic behind the solution of the mystery worked through in a way that makes sense--there are leaps (with a few clues dependent on assumptions about 1930s gender roles, say) but always an "if this, then only that" logic that, in retrospect, makes perfect sense. So it lives up to the fair-play reputation. Also, though Golden Age mysteries are often dinged for being figuratively, and I guess occasionally literally, bloodless mental exercises, there's a commendable focus here on the characters' horror of death. Alongside this, which I know is contradictory, there's also a great deal of meta- ingenuity in the multiple, inventive narrative structures of the stories and the kinds of clues on offer--a frayed rug, a bearded lady in a painting, a hotel's name, someone buying six identical cats, each a day apart--that mixes things up quite pleasingly.
The extra-textual parts are the ones that I want to ponder. Martin Edwards's massive The Life of Crime quotes one of the authors, cousins Frederick Dannay and Manfred Lee, who wrote together, by his account and Otto Penzler's not especially harmoniously, as "Ellery Queen," claiming that Queen the character was essentially Jewish. There's a double masquerade, with the two authors, birth names Nathan and Lepofsky, masquerading as WASPs masquerading as Queen. And I just don't see it, having read this collection and at least four Queen novels. Feels like the opposite to me, a fevered stab at assimilation. (Not sure I've encountered anyone in this period writing mysteries who was Jewish. All I know is Israel Zangwill's The Big Bow Mystery, which came out in 1892. I wonder if this was because a lot of the writers who might have otherwise have written pulp/pop mysteries considered the form too ideologically conservative and wrote proletarian fiction instead.) The most common verbs for Queen's speech here, aside from "said," are "drawled" and "murmured," both of which cropped up at least once a story each and several times more frequently--the seams usually show up somewhere in old popular fiction--which feels like an aspiration to the upper-crust ease of, say, Willard Huntington Wright's insufferable Philo Vance. (Read all of those novels a while back and am leery of going back.)
And the cast of characters here is, uh, narrow--there's none of the anti-Semitic stereotyping you often run into in period novels, because there are no Jewish characters; though there are, unfortunately, a couple of extremely secondary Black characters, both of whom speak in dialect, and one German, who I'm fairly sure isn't Jewish but could be, and everyone else is named something like Gardner or Shaw or Sherman. Plus, Queen is frequently given to dialogue in an exceedingly Vance-like vein, which Otto Penzler says mostly came in the early stories (would have helped to know which was written when). Such as: "'Bless your orderly old Teutonic soul,' said Ellery. 'Velie, cast those Cyclopean peepers of yours this way.'" Or, after Ellery tastes a cookie and likes it, "Yours? Delicious. A veritable Lucrece, b'gad. Or is it Penelope I'm thinking of?" And later, "Don't be alarmed, Marie [not her name]...this isn't an abduction. You have achieved Valhalla." Also, wow, you could do one of those Twitter threads about how often the female characters' busts are mentioned. That, too, was pretty much every time. So...ace period detective fiction that's very much of its period.