I've dabbled with 3M (despite using Linux and not actually having an ereader that supports EPUB, let alone ADE, so I had to read on my laptop).
What I found is that selecting a checked-out book will try to download it, not open it for reading or anything silly like that.
It is basically a frontend for the checkout desk, but you still have to have ADE in order to read anything.
I did not witness it downloading all your books when you selected a different book, I just saw it redownloading duplicates of any book you are foolish enough to accidentally click on.
...
Overall, not impressed. And not impressed with their catalog quality either.