Let me try some use case questions along with tentative answers:

Does a high speed card make any difference:

Q. On a stock machine doing web browsing, regular apps?
- no

Q. When showing video?
- no

Q. When copying files from a host PC over the USB connection?
- yes, but within limits

Q. When copying files directly to the card inserted in a USB reader (ie. no IT involved) ?
- I suspect that this is the big win - especially if you want to copy big files (movies) to your card.

Q. On a tuned kernel booted from a high speed card?
- Yes, worth having if you are happy to fly this way.

Please confirm or deny my answers.