We have now been using Kindle Select promotions for 15 months, across 150+ books.
And, I have to say, it is a big winner. We still have a few books we publish across several e-book retailers, but 95% of our books are exclusive to Kindle Select.
There is a publishing snobbery about Kindle Select - "It is only for self-published books", that sort of thing - but getting books in the face of potential buyers at the point of sale is always a good thing.
Which isn't to say it is magic. Some books resolutely refuse to be bought. Some are inconsistent in their responses - in fact, they all are. But, across the board, using Kindle Select promotions has massively increased our sales.
Books like 'Mrs Jones' and 'Hooking Up' shot up the charts almost immediately. Others, like Drew Cross' 'Grind Their Bones' and 'BiteMarks' took a year before anything happened. But, as I say, most books sell many more copies having been promoted on Kindle Select than if they hadn't.
Yes, there are hazards. Reviewers of books obtained for free are, for some reason that has yet to be explained, the harshest critics around. We call it 'freeloader guilt'. Plus, many authors hate the idea of giving their books away, even if it means they sell multiples more than they ever would have done otherwise as a result.
So, our considered advice is to try it. Play with the promotions. Play with the pricing. An electronic market is a real market you can approach as if it were a Simulated Test Market.