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.