I setup my Canon EOS 1DsMkIII camera on manual mode, continuous high speed shutter, and clicked away. Here is what I learned.
20 shots from the first shutter release until the 20th frame was fully written to the card (red light stopped illumination)
Sandisk 8GB Extreme III = 25 seconds
Sandisk 8GB Extreme IV = 18 seconds
Ok, so I thought I would stress the cards just a little bit more.
30 shots from the first shutter release until the 30th frame was fully written to the card (red light stopped illumination)
Sandisk 8GB Extreme III = 38 seconds
Sandisk 8GB Extreme IV = 25 seconds
This test was one of those informal tests that aren't all that scientific, but somehow give you satisfaction that you accomplished a goal for the evening. What did I learn? I learned that the Extreme IV cards are worthwhile for me to continue to have in my bag. This speed in crease comes at nearly double the cost, however. I would like to wait until Sandisk comes out with 16GB Extreme IV cards, but I am not sure how much longer I can wait.
Your mileage may vary.