Last night I formatted the card again and ran it 'til it failed. the first time it failed after the second shot. The second time it failed after six shots. After formatting it the third time, I had to run it out to almost 700 shots before it failed.
I vaguely remember that with a computer hard disk, any bad sectors are marked so that the computer doesn't try to write to them. Could the same thing be happening here?
As far a speed is concerned, it comes in handy if one is trying to make a series of continuous shots. I was trying to get birds flying between a bush and a bird feeder - a distance of about 3 feet. I set the camera on a tripod, aimed the camera between the bush and the feeder and set the focus to manual. Then, as soon as I saw movement, I woulld fire a 3 to 5 shot burst. Got some great pix, but missed others while the camera was writing to the card. That's why I wanted the faster card.