I also prefer SanDisk cards. My main purpose for ordering cards is to replace my current ones with 16GB and use the current cards as backup. I do have backups now but they don't have near the capacity.
My main purpose for this query is that Nikon seems to be behind the times on memory card approvals. Example, there are no Extreme IV or Extreme III SanDisk cards, among the ones approved by Nikon, available unless one goes to say, eBay or Craigslist or similar sites. It's Extreme or Extreme Pro now. So I was wondering what people use these days.
So the discussion on card degradation is probably a moot discussion. As you say, they either work or they don't, apparently. I know there is a check digit, computed through a standard algorithm (in this case a memory card process algorithm), added to each character written to a disk/card. When the character is read back and the check digit recomputed by the reader that check digit should match what was originally written. If it doesn't then that character is corrupted and therefore dropped. So, in imaging, a dropped character would mean a missing pixel(s) on a screen, which would be somewhat apparent on the screen/print. That is the basis of my "image degradation" question. If at least one character becomes corrupted does the whole card fail?