[Dabhand16]

You have hit the nail on the head, it is the read/write speed.

Sandisk are up to Extreme 4 now, Ultra 2 are relatively old with the Extreme 3 being brought out after the Ultra 2, but to get the full capability of these cards (Extreme 4) you need a special card reader to download into your computer.

These high speed cards really only give advantages if you use burst mode/continuous shooting a lot as they will allow the camera buffer to be cleared faster, and/or if you need to download pictures into a computer quickly. Sports and action photographers are the people who will reap the most benefit from them.

All of the major makers are now producing these high speed cards, and as prices continue to fall, most people are buying them when they need to buy new cards.