I just bought a Lexar 800x 64Gb CF card from Adorama and put it in my 5D Mark III. I haven't taken it out. I've been transferring via USB, then formatting the card in camera before I shoot again. Twice now, I've gotten corrupted files, and once a shot that I really wanted just disappeared as I was scrolling though the photos on the camera. This is the first Lexar CF I've ever bought. I've only shot with Sandisk up until now. I think I like Sandisk better.
I just returned two 32 gb Lexar cards to B&H. Got error 80 that locked up my cameras (5DIII and 1DIV) while shooting basketball. There is apparently a known problem with 32 gb Lexar cards and canon cameras. Glad to hear your 64 gb cards are working. I am going to stick with Sandisk from now on.