There have also been cases where a customer violated Amazon's terms of service and lost access to all of their Kindle e-books. Amazon has all the power in this relationship. They can and do change the rules on us lowly peasants from time to time.
Note, there are indeed ways to download your books and import them into something like Calibre (and remove the DRM from the books). If you do some web searches (and/or search YouTube) you can probably figure it out.
The interface is 100% of the reason I won't use it. It's by far the worst experience for navigating a library I've ever seen. It's just access to your filesystem, except with effectively no files on the screen at a time.
There's no tags, no ability to choose between by author, series, publisher, genre, etc, just a really bad presentation of your filesystem.
Everyone is entitled to their own opinion. I personally like it, even so my main reason for recommending it is the fact that it has many features and supports a wide range of ebook formats, more than the stock Ereader apps usually do.