Just 14 seconds off my fastest Thursday ever, and almost a full 13 minutes faster than my average, this puzzle was over before it began. The theme, XMARKSTHESPOT (15D: Treasure hunt phrase ... or a hint to seven Across answers) was a dead giveaway, and the only temporary hold-up was the strangeness of a (very good) non-theme answer at 28A: Rug rat (ANKLEBITER). I know that the theme answers are not symmetrical, and I like that, but somehow, as the other, longest Across answer, I just expected one there.
I enjoyed STIFFARM (17A: Push aside for selfish ends), AMBIENT (42D: Surrounding), and GASLIT (7A: Like London streets during Dickens's time), where I first tried "filthy." Hah!
Entries I did not like included ETALII (52A: List shortener), RANTAT (21A: Harangue), and my least favorite, of the week, and maybe the month, is UNTUNE (45D: Affect, as humidity might a piano). That is not good.
1A: Calvin from "Calvin and Hobbes," e.g. (IMP) - C. I tried "boy." Heh.
Favorite: ESPRIT (43A: Sparkle). I just like the spirit of the clue.
Least: See above.
The X/SPOT theme was fine, and well done (I especially liked the double in PRIXFIXE), but it was just too quick for a Thursday.
I got stuck looking for other Xs where none existed. I like that every X in the puzzle is used in a theme answer. I wonder why the entire SW corner is X-less. I too tried "boy" at first. UNTUNE is the worst, for sure, but ONATEAM is pretty lousy too.
UNTUNE is, indeed, awful. And I agree with Colum about ONATEAM as being pretty lame. But I'm OK with ET ALII, because it's both spelled out and is the much less common masculine specific variant of the admittedly-crosswordese "et alia." Although, even that spelled out would be OK, I think. BTW, isn't it kind of silly to save only one character by abbreviating it, "et al."? OK, I suppose two characters if it's at the end of a sentence and the "." does double duty ("..., et alia." vs. "..., et al.")
Looks like I finished last today. Took me too long to figure out the theme. It fell quickly after that. I wanted it to be a rebus, but couldn't figure anything that worked in both directions.
Loved the section with GAGREEL (7D: Closing credits feature, sometimes), AGEONE (16A: What precedes a terrible period?), and TEXTER (12D: One who shouldn't be driving). Nice clues all around in this one.
Correction - I finished last. I was hung up in the NE with GASLIT because, like Horace, I was thinking filthy and I couldn't come up with the GAG of GAGREEL. It didn't help that I kept wanting cameoX instead of GUESTX. The rest, though, did go a bit fast for a Thursday.