I can't speak to countdown deals, but as an experiment last year I ran a five day free promo on my book 1 (it's in KU, and I thought - why not?) without any ads or newsletter promos.
This was in August. I got about 140 downloads, and a spike in KU page reads - I'm not a big seller, so this was a nice rank boost, and didn't cost me a penny.
Previous experience of running free days is that I've always booked newsletter promos for days 2 / 3 / 4 of my 5 free days - I'm paranoid about getting the times / dates wrong across timezones. I've always seen a bunch of downloads on the first free day, with no promotion.
So, I think that there are readers out there looking for a good deal.