Thanks to ColinsMum and everyone else who recommended this.
It's become clear that DD6 is being held back from further conceptual progress by not knowing her math facts. (This kid has never met a word she can't fast-map, but she can't remember addition facts? I don't get it.)
With previous math programs, DD has operated at one of two speeds: Too Easy, or Hysterical Meltdown. With this, she seems to be finally hearing me when I say things like, "It's okay to make mistakes," or "Don't stress about it, just move on to the next one." She does sometimes get too frustrated, and I make her stop, but unlike previous hit-the-wall situations (e.g. Dreambox) she actually wants to go back to it. Repeatedly.
Best of all, she seems to be actually retaining some of it!
Knowing DD, this may only last a week or so, but I'll take it while I can get it.