My experience was similar to pop's. A month before the release of my second book, my publisher ran a promo making my first book free at Amazon, Barnes and Noble, iTunes, etc., and ran a BookBub ad for it. That book got up to #2 on one of the Amazon bestseller lists (free ebook -historical romance....and up to #25 for free ebook fiction).
I don't have hard numbers yet on the effects of the promo in subsequent months when the price went up again...nor do I have numbers yet on the sales of book 2.
But I'd say it made a big difference. (Since it was all done through my publisher, I have no idea how much it costs to work with BookBub.)