People tend to only think about one end of card, which is the write portion. Don't forget the read speed as well. Case in point, I recently tried my K5 out with an old 2GB SanDisk card (unknown speed). Shooting was ok, as I wasn't pressed for time and only took a couple shots goofing around. The camera was set to highest JPEG setting, and transferring the shots to my computer was quite slow. I'm not talking about days here, but it was slow. A few days later, my new 32GB class 10 arrived and I popped it in and took a few shots. Up load time to the computer was amazingly fast, even considering how big the files were.
What difference does upload speed make, since you can walk away or multi-task while the files are transferring? Just this: in the past, I have noticed, especially on my laptop, that the longer it takes files to transfer, the more risk I run of having a problem with a corrupted file. In fact, that was what started me coping files instead of moving them, because if I had a hiccup during the process, at least I didn't lose the original file (a painful lesson I learned the hard way). This class 10 is so fast I'm almost tempted to going back to moving files again. Almost...