Have you used the new KindleGen tools yet? Well, there's an interesting quirk to the tools that isn't readily apparent from the outside.
When you make a KF8 ebook, the output will actually contain up to 3 different ebooks, not one. Seriously, inside that ebook will be one for KF8, another in the regular Kindle format, and if you use an Epub as the source that will be thrown in as well.
As you can imagine, the output is going to be a rather bulky file, and that could be problem. Size doesn't matter if you're submitting the ebook to Amazon (they'll fix it for you), but if you plan to give it directly to customers then there's a problem.
Let me give you an example. One of the O'Reilly books I own is a 43 page fact book. That's a small book, so you would think that the ebooks would be under 100K, right? Well, no. The PDF is 2.3MB, which is not unusual. But the KF8 ebook is 16MB, and that is ridiculously large.
FYI: Kindle (and Epub) ebooks are usually smaller than the related PDF, not 6 times the size.
Now, I might be the only one who cares, but I think everyone should. Those large files are a waste of both bandwidth and storage. All the current Kindles have a finite amount of space, and handing a customer one of these files effectively reduces the storage by 90%. Needless to say, that's not good.
What's more, it's also not necessary (right now). KF8 is currently only supported on the Kindle Fire, and if a KF owner wants the better formatting they can get the Epub and read that with another app.
So if you're making and selling your own Kindle ebooks, please reconsider not using the new format. At the very least, I would wait until there is a less bulky option. Calibre will eventually support KF8, and it will probably not make such a mess.