Just to prove this wrong, I had a class 10 card sitting around (16gb SanDisk for GoPro), and I have some Samsung 40MB/S write and 80MB/S read 16GB cards which I've tested previously against the San Disks and they do just fine. So the test was two 15 minute videos 720 at 60p - GoPro, and 400 RAW files from a D300s, coming straight off the card on a SATAIII cable to a partitioned RAMDisk. The only bottleneck occurring at the card and the total size of the transfer being 6.21GB for the RAW files and 3.15GB for the video, totaling 9.36GB. The times are:
45MB/S Samsung card - 2 minutes and 31 seconds - it was slower with the individual RAW obviously, but average of 62 MB/S read so not too bad.
Class 10 SanDisk card - 6 minutes 3 seconds - slowed down a little again on RAW, but avg of 26 MB/S read.
So if anyone else would like to argue still about how there is no real difference in the cards, I beg to differ. the difference is substantial, and I know it saves me a lot of time when I come home and have many full cards I need to dump, and there's not much you can do other than sit and wait, and personally, I would waste a lot of time and productivity on class 10 cards. So eheath when you argue that there's not much difference, I'd say there's more than twice as much.