My experience of free has not been especially great - I've experimented a number of times with making one of the short stories from The Other Room collection free, and whilst it's resulted in a lot of downloads, the resulting reviews have been along the lines of "... it's a short story I feel robbed..." which is slightly disheartening.
When I've done promotion activities actually targretted at people in the horror/weird fiction community the result has been much more postive, both in terms of reviews and sales. So not sure I will do anymore Amazon freebies, although who knows?
That said, if any of the people of impeccable taste on this site want a free ebook of the story, let me know....
Or even the full collection.