I've completed the 13-level demo last night. It was pretty enjoyable. The gameplay is excellent. I think I understand the behaviors encountered on all levels except the last one--the exact outcome as you move baba(s) around under the initial rules state is still fairly unclear to me.
$15 still feels a bit steep a price, considering the simplistic nature of the graphics, sound and music (I understand it's a purposeful stylistic choice on the part of the game designer, but still, it's one less thing to justify a higher price on). So most likely I'll still be waiting for a sale of some kind before considering buying the full version. Did they say anywhere how many levels the full version has?
Through testing, it looks like the demo actually will cycle through any level files it finds in its directory, it's not hard-coded to 13. Makes me wonder how easy or hard it might be to reverse engineer the binary formats of the level files and thereby potentially create your own levels to play using the demo.