[Pinkie]

I can't get the Cloud, any of them, to work seamlessly for me. Just can't, so don't even. I've had so many problems, and it takes so damn long to do anything (and we have superfast broadband, but the free accounts are throttled at source) that I've given up.
I use my Kindle Fire for most of my reading, and I've installed Google Play on it with the simple hack. But Calibre Companion on the Kindle Fire has severe problems connecting wirelessly with the main Calibre library, and so I've given up using CC. It's fine on my Android phone, but because it's so bad on the Fire, there's not much point using it (I blame the Fire rather than the CC app, but the result is the same). If there was a way for CC to see and read files loaded on to the Kindle Fire by cable using Calibre, that might be an answer, but still... for now I use Moon+ Reader as my library.
Like most mortals, I only have 2gb on Dropbox, and the extra space is really expensive, so I just use Dropbox to sync. I have my Calibre Library backed up on gmail, as well as local storage, but I don't use it to sync wirelessly as that doesn't work. The library is huge, so no free storage can cope with all of it.
I used to use Mega, but it was once owned by Kimdotcom, who is currently in prison in Australia, so I've discontinued it. I just don't trust it.
So that's where I am currently, hooking my devices up to the main Calibre device by cable. If I use my other devices (I have laptops for travel) I use my local copy of Calibre Library, which is on a portable hard drive. It's not ideal, and it's not slick but at least it works, and I have reliable copies of my precious books.
I'd love to have an Android library app like CC which can read the books in the main Books file of the device however they got on to it, but I haven't found one yet.