My family is a multi-Kindle family. My husband and I each have a "real" (e-ink) Kindle and we each use a Kindle app. He has an iPad and I occasionally use the app on my Android phone. My dad has one of our old Kindles, so we're bumping up against the six-device limit that Amazon allows on one account.
That's not really the problem. The problem is that Amazon makes your latest (most recently purchased) Kindle into the default for your account, the one new purchases will go to unless you change the setting before you click the buy button. There doesn't seem to be any way to change the default Kindle! I really think they need to let you click the buy button and then make you select a Kindle to send the book to.
I keep sending books to the wrong place! It doesn't cost me any money, but at different points in time, my husband and my dad have both gotten books I know they don't want. I have two good friends who write romance, and I can tell you neither guy is interested in their books.
So Amazon, listen up! Change the buy button so if you have more than one device registered, it makes you pick a Kindle after you buy!