My only experience with the EyeFi cards was, to say the least, disappointing. That was with my 1Dx. I've yet to use the EyeFi on my 5Ds because I can't imagine it is capable of coping with or pushing the data stream the 5Ds requires. It was kludgy and slow on my 1Dx, plus I hated the interface to use it. Granted, this is my own bias with the EyeFi and you may have had better experiences. The times I tried to use the 1Dx with the EyeFi, the EyeFi/iPad choked, so I can't image it's any better with the 5Ds.