I had the 64GB SDXC as well, it at first seemed to work, but don't be surprised if it stops working. Motorola, and I believe stock Android do not support SDXC yet. I have a somewhat detailed thread here: http://www.droidforums.net/forum/dro...b-vs-64gb.html
When I got my card, popped it in, it formatted correctly, and seemed to work fine. As I started to put it through its paces it started to just not work. On ICS when you used the Storage option to transfer media to the SD card, it would stop halfway through and throw out errors. Eventually certain apps would stop writing/seeing the card like backup apps etc..
Be careful going forward, and test it hardcore. Transfer files back and forth, I always had issues with movie files. See if Root Explorer or other explorer apps can see it, see if Titanium Backup or My Backup Pro/SMS Backup can see and write to the card.
And yes, I did not get a dud, its works in other non-Android devices and works and tests perfectly in multiple computers. Just be cautious.