It kind of gives you a twofer--the Kindle book and the audio, too, and you can listen to both on Kindle (or mobile phone).

I just tried one--the Kindle book was $11.99, and the whispersync was $12.99, so altogether the book cost me $24.98....

Kind of steep :(

This is a non-fiction book--I enjoy listening to audio books, but I think this book will have enough facts that I will want to refer to the print version as well.

I would have paid $25 just for the audio version had I ordered the CD.

I haven't tried whisper sync. But I did see a notice on one physical book I bought that I could get the ebook copy for 99 cents. 

DH loves audio books, but he isn't into the stuff I read, otherwise I'd give it a try.

I use it tho Whispersync doesn't seem to work right on my K3.  I actually use Imersion Reading more!  This seems to work only on my Fire HD tho.   You can get them much cheeper.  I got 31 Free Classic when they 1st introduced it.  Once in awhile I add classics they offer ..some free some 99 cents for  ebook with Audible Free! 

I have but only when both were super cheap.  I would not pay basically full price for a book twice. 

The couple I did were where I downloaded a free classic book and then the audible book was only .99-2.99.  I think I did Moll Flanders, A Portrait of the Artist As a Young Man and Heart of Darkness that way.  Most times when I see that offered it's too expensive to me to bother. But then I mostly only do the free ebooks.

eta: I never bothered to read the kindle version, I just listened to the audiobook so I never tried the syncing thing.

I like to listen to audio books. If you have the kindle version, the audio on Audible is much cheaper, often much less than 1 credit, like $1.99 of $2.99 or $3.99. The prices are set by the publisher, though, and sometimes it is $12.99! Still cheaper than 1 credit, and sometimes they even have sales. (I am an Audible member)

Here is one advantage I found: I can BORROW the Kindle book from my library; it goes through Amazon, then Audible sees it in your Amazon account and gives you the discounted audio book at the reduced price! Even after the library loan expires. 

Since I don't have a Kindle Fire, I haven't done the switching between reading and listening; I just like to get the audios (and listen on my MP3 player). 

Amazon also offers to check your kindle library for books that offer Whispersyncing (It's somewhere in the Whispersync links) and tells you right there what the discounted audio book price is. I love that!

I only get audio books from the library. I can't see paying for them. They are usually way too expensive. And I wouldn't want to buy a discounted one to go along with my ebook. That makes no sense, to me. One or the other!