I'm using them with my 1Ds. Can't say I've noticed a major increase with transfer's within the camera, but then I don't do high frame rate shots.
Where they do score is transferring to the computer. I'm using a Lexard USB reader. However even with a Firewire 800 reader, the speed is only 10% faster. The bottleneck still seems within the card itself. However having said that transferring some 8Gb of RAW files is a lot quicker than with a slower card and a conventional reader