While the original Kindle Fire existed so you could buy stuff from Amazon, the new models exist so Amazon can pitch content and sales to you. Do you see the difference? While the older model is a consumption device, the new ones are advertising devices.
It's not just that the lock screen has ads; Amazon has also rigged the KFHD so you cannot easily replace the lock screen with a different one. I've tried several, and Amazon's ads have not gone away. And there are also the ads at the base of the home screen, something we knew before the KFHD shipped.
But those aren't the only sales pitches with which Amazon bombards users. At the bottom of the home screen is a second set of product pitches labeled "Customers Also Bought" where Amazon displays more content for you to buy. Amazon has also rigged the KFHD so you cannot easily replace the home screen (unlike the original Kindle Fire) so they're not going away even if you pay the $15.
Clarification: I've found and installed several alternate home screens. The KFHD refuses to allow me to open them - even though they validly installed.
Note that I'm not saying that you should or should not get a KFHD, but be warned that this tablet belongs to Amazon, not you.
One reader wanted to know why I bought the KFHD but wouldn't buy the Nexus 7, even though they appear to fit in the same basic niche. I got this tablet so I could take a look at Amazon's business model. Thanks to the advertising the KFHD is really not in the same product category as the Nexus 7 or any other Android tablets. While they all run Android they were designed with very different goals in mind.
It seems to me that Amazon has taken B&N's idea for the Nook Color/ Nook Tablet to heart and built a walled garden where Amazon can constantly throw advertising at you. Frankly, this isn't a media tablet anymore.
This tablet doesn't exist so you can do stuff and it doesn't exist so you can consume content. The KFHD is in your hands just so you can be advertised at. If you buy one you need to keep that in mind.