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.