Here is what I've found:
Twitter ads DO NOT increase sales
I found that I was getting the usual purchases throughout the time span, and at some point during those days, their ranking increased (it's not a good thing).
However. The rank increase could be because a lot of books launch on Tuesday, pushing my books from their usual rankings. By the end of the week, they resumed their usual positions.
While the ads didn't generate profit, I did get some new followers. So it did grant me certain exposure.
All in all, the pricing was decent but it didn't get me the increase in profit I wanted. On the other hand, I did attain followers. Depending on your goal, it's something to consider.
I think they can absolutely work, but because there are so many of them, people don't even look at them even more. I know I don't.
@booktweeter also provided a copy of each tweet they sent out and any retweets, which I really liked.