My books are sold through D2D so I had never done a direct store promo before. You have to ask or be invited to these. Well, I was finally invited by Apple (after prodding D2D about any avail promos a few months ago).
I was disappointed for the first two days. By the second day, I sold only two books. But I'm at thirty sold now. The push was around fifteen sold today (I'm guessing from Apple's own advertising and it's the last day). That's impressive for a 99c promo--like ENT results for me.
A few caveats. I dropped my book price to 99c. Many other books are actually trying to make money

I added an ad push of $10 to move the book higher up on the carousel (as I've done before with Kobo). I made the cost back in 2 sales for the second book. I think the promo actually pushed sales from zero initially as the book was first selling at the bottom of the pack.
Biggest negative: Amazon price matched the book, of course. This is always a risk and now I'll have to contact them.
The best thing to do would be to pair these with a standard promo but timing these things can be very tricky.