Interesting, I generally have a fairly positive opinion of the Imp. In particular the integration with Squirrel means that much more complex things can be done, more easily, and without touching the running hardware. The Spark (Core) on the other hand is much like an Arduino in that you have it in your presence to code its behavior, and then release it to the wild. Slightly different target apps, hugely different user experience.
That said, this is a pretty interesting price point. I find myself unable to resist ordering a couple :-).
Oh, and to answer your question, when folks say "Spark" they generally think of the "Core" which was $39 (or twice as much) so this really is saying, "hey a new cost reduced version of the Spark core."