When I need absolute speed on my 5DmkIII, I'll just use my 1000x CF's with nothing in the SD slot. But having dual format slots does come in handy sometimes. I can shoot RAW to CF and JPEG to SD and give the SD card to my GF so she can have the pics without waiting on me to process them(she's not as picky as me and her MBP has a built-in SD slot) and I can also use the SD/JPEG with the camera/usb kit on my iPad to preview images. Or I can shoot RAW to the SD as well and use the SD card in my RMBP and not have to pull out my USB 3 card reader. But I will say, I prefer a more robust card like CF.