Normally, I've never had difficulty with SanDisk products, especially since I have a microSD in my New Nintendo 3DS XL, but I have been hearing the difficulty some have had with the product when it comes to storing personal videos and photos. For some reason, when you turn the Kindle Fire off (as opposed to just placing it in stand-by mode), then turn it back on, the Amazon Storage SD system will apparently move your downloaded photos and videos into the Others file for some reason.
Meaning the files haven't technically been removed as you might suppose (since the SD Storage will still acknowledge the memory is still there), but it does make it become rather inaccessible from the Kindle. Technically when I connect to my computer to locate the files I'm not sure how to find them, or even if i can move them back to the appropriate spot.
Overall, the result is usually having you to reformat (aka erase the card), then putting back your personal items on it via backup. Which for me is not really a pain, but having to reinstall apps and restore Amazon Books and Films back on the card can be tedious. So you're probably saying, 'Well, don't turn off the Kindle then.' Consequently, this can be difficult though when you have to do a power reset, like, say you're Yahoo Account is having problems, or you accidentally let your Kindle decharge completely.
I'm wondering if the problem does not lie so much with the memory card, since it hasn't actually popped out my files or reshuffled them to Others unless you shut down the tablet completely. But instead possibly something with the Amazon OS which somehow either transfers the files or just makes them inaccessible upon powering up.
Overlooking this obstacle, the SanDisk Ultra is a pretty good microSD Card for what it's worth. Hopefully Amazon, or SanDisk themselves, can look into this problem and can rectify it with an upgrade or a way where the card is formatted with the appropriate files you can use to place your photos and videos onto it.
As for the book issue, the SD Setting menu claims it will allow books to be saved to the card, however, as with the RiffTrax downloads, the Comixology books will not download to the card for some reason. Perhaps I suspect this is due to the fact these are Third Party services (despite Comixology being part of Amazon) and it defaults to the device storage instead. Still working on it to see if there is a way at least for Comixology books to be saved to the card.
I've heard on the lower reviews that they've gotten a PNY 128GB MicroSD, and plan on getting one just to see if the issue is either really with the cards themselves or the Kindle. So far the card storage system works for what it's worth, but like I said, until they can find a way to fix the issue of it shoving the personal files into the Others, I can see this becoming rather annoying fast. My only current suggestion is, to create backup files of your stuff in a separate 'Kindle' file on your computer and just transfer it back onto the card whenever it does this (after erasing it). Hopefully I'll let you know in an update how the other brand works and if it suffers from the same issue.
UPDATE: Adding a star because I did solve the Amazon memory issue. Basically I discovered that I was writing folders onto the card through PC. But here's why it went wrong, by doing this the FireOS will not recognize the folders as part of its storage system.
In short, when you power down your Kindle, it will automatically dump your personal folders and data into the dreaded OTHERS file. So here's what you have to do to prevent this glitch:
Connect your Kindle to the PC (via USB cable/charger). Go to the Kindle icon in the Folders Menu, then into Internal Storage (aka Kindle Hard Drive). Locate the Folders marked Downloads, Movies and Pictures. COPY (don't MOVE) and PASTE these folders into the External Storage (i.e. microSD card). By doing this you will have formatted your card where the FireOS will acknowledge the folders and the contents as part of the Amazon storage. So if your Kindle does accidentally power down, it won't dump your personal stuff into the Others file, making them inaccessible to your Kindle.
However, this isn't foolproof but after doing some tests after the above procedure, I so far haven't lost my personal videos and photos on the card.
Hope this might help you out.
Wallflower 
Sarah
Michael Jennings
Dunestar