This is full of numbers so you know I can't/won't do it, but here we have James Lewis breaking down his experience with Kindle Select and the additional promos he did for his non-fiction book. James Lewis.
I can say I continue to be exceedingly pleased with the results at GoodReads. Maybe because I can SEE them.
You can see if people added your book to their TBR list. That doesn't guarantee they will buy them and read them but at least you know they've noticed you.
Why can I say that when I just gave away 1000+ copies of Schtupid Cupid? Didn't 1000 people notice me? Um. Yes...they downloaded a free book. Okay, what's the difference? The people who added Murder to their TBR list will have to buy it and they expect to read it. I have no idea if the people who downloaded the free books (and I'll say that for Smashwords, too, there have been no sales of Flash 1 or 2 after the freebie week) will ever read them and fewer still will ever review them.
All this is something each author--ick--writer has to test for themselves. It probably is different for every book and maybe even every month. Summer may be one thing, Winter something else. Take the experience of others, weigh it and try to make an informed decision on how to proceed.