I initially purchased 1 KomputerBay 64GB 1000x card for my 5D3 ML in May 2013, and it work perfectly.
Then I picked up 5 more of the same in the middle of June, and they were even faster!
We filmed a short with all 6 cards, gathering 2.68TB of footage in total (downloading from the cards 42 times) and we didn't have one single issue with anything! There weren't even any dropped frames.
I did notice, however, that after like 10-12 times through, the cards would initial write at a slower rate and then quickly pick up to full speed.
For the price, I think KomputerBay is the way to go... and their new cards make it seem as though they got the message, and want to be the cards used for ML.
www.davekratzfilms.com