I just got two of the Lexar Professional UMDA 7 1000x CF cards that B&H are running a special on for a two-pack. Wow!
Testing in the 5DIII shooting full size RAW, set to high speed continuous 6FPS, I did a burst and shot until it started to splutter. Would you believe 35 frames, and it cleared for another full run after about three seconds.
The same test with Sandisk Extreme 60 Mb/s 32Gb, it started to splutter at 18-20 frames, and took way longer to clear. An older 8Gb Sandisk Extreme IV 45 Mb/s carked it at 15 frames, and understandably took longer again to fully clear.
So if you are consistently hitting buffer in your 5DIII, get yourself a couple of Lexars. I'm very, very impressed.