I've just set this up and i kinda worked, but i had a crash with xbox-msuic after downloading an album. So i just have an eye on this. If this turns out to be a stable solution (and the crash was just this .... xboxmusic), i'll edit the faq, and i could ceep he 32GB-version instead of the one with 64GB.
Kinda depends on your behavior. I'm running two account on my 32GB, and yes there are 2 GB gone, but as long as the apps being around 50mb, I will have a hard time to get the remaining 13 full. But in long terms, maybe 64 is the better number
I had no trouble at all with the class 10 sandisk 64gb card I got. Also, rather just adding the mapped SD folders to the libraries, I MOVED the entire main documents, videos, pictures, and downloads special folders into the mapped SD folder. This means that all my media will be stored on the SD card (which sounds fine to me), and it seems to be a less buggy solution that using the libraries split between the two destinations. This is the same technique you'd use on Windows 7/8 to relocate your Documents, Videos special folders to a different drive.
You can right click on the special folder, choose Location and pick move-- or, you can actually just drag the folders and Windows will take care of the back-end automatically.
This has worked pretty well for me, although I haven't been testing it for more than a day.
As an aside, I really dont see the point of being iconoclastic about 32 vs 64 internal. Both are on the market, and the SD card issues need figuring out one way or the other.
Since i have to make a decision between 64 and 32Gb here's a problem i'm asking you to follow up: When i'm playing files (esp. music) from my card (linked with the method by radicus) and the surface changing to connected standby/screen turns of, every app stutters or completely stops playing. The only workaround (for using the sd-card) with a 80% reliability is streaming via play-to.
Playing from internal memory or a usb-drive is no problem.