I ran bargain books on KND since I signed up forever ago. I honestly can't tell if they do much. The position on the page was moved late last year and its no longer highlighted in a good spot. I think they are throwing too many things at the readers now to be effective. I have one more slot later this month.
KFD rejected one of my titles (contemporary romance) b/c they were uncomfortable listing it. KND will list anything. They're a business. BB is a business. I got the impression that KFD is a person doing it from their kitchen. Some ladies were yelling at her for posting some book that was mainstream b/c it offended them. She was trying to calm them down, but they were crazy with a K. There is no way to know what will offend and what doesn't. I felt kind of bad for her. Anyway, I don't submit over there anymore b/c of that. I like to have all the rules up front and consistency.
I'm hoping more stuff like BB pops up. They have a firm grasp on successful marketing do's and don'ts. Plus it helped with the other vendors, not just Amazon, which was awesome.
I've been hearing the cover reveal book blogs are cheap and create a lot of buzz, too. Might want to look into that option as well for YA or romance.