It works a treat for some; for my books, not so much. Today at midnight the Verrine book finished its stint. Make sure you go in and change the settings before the time is up otherwise you will be automatically resigned. If you don't check the re-up box, you can think about it and they will send you a note telling you how great a program readers think it is and that you can be a part of all the money to be had in their fund. I made a pittance. People don't want to borrow cookbooks.
Do you get extra special visibility? Where is that? I have one book on that I intentionally did not run a freebie on and in close to 2 months there hasn't been 1 sale. Kindle Select isn't magic. Like in the real world, you have to start with something people already know they want to read. If you have a niche audience, if you're off-genre, if it's not something easily understood, it doesn't look to me as though Kindle Select is a help. One might say that it's a hindrance because the more sites you're at, the greater the possibility people will find you. Not everyone goes to Amazon, and those who do are of a type. I'm not sure what that type is except that I know the BN audience is a different type.
I can't argue for or against it. If you have 1 book, does giving away a ton of them help or hurt? I have no idea. If you have a series already, does having a freebie on book 1 help or hurt? All I can say is that hundreds of copies of Bad Apple 1 were downloaded and there was no spike, no thumbtack in sales of it or books 2 & 3.
Write something we already know people want to read. That's your surest path to strong sales.
So Kindle Select is both pass and fail.