Regarding your questions, which would demand some knowledge of the respective field, I can't say much.
My personal experience was thouroughly positive. I've noticed instantly (after around 10 s) states which are very similiar to those of meditation. I guess, that would be theta waves. Now while I experience such during any form of pointed concentration rather soon, your app has done so very fast. Gotta try it tomorrow, to see if there is a proper causation.
A suggestion on the side: As a variation, I'd find it interesting to read more 'neutral' words, in anyways no story, yet words, which are of the same semantic cluster or neighboring. Superficial 'neutrality' of neighbouring words appearing might allow the mind to create deeper, not too obvious associations between them, while having not to struggle with a disperse series of mental pictures. I think, that, additionally, a creative process could be induced and enjoyed. A story utilises much of our attentive space, which could be perhaps used for the construction of such.