To the OP, Yes. A big difference. I have a 16GB Patriot that gives me intermittent memory error messages on the LCD. A power-cycle on the camera fixes it, but I don't use it much anymore... not for client work anyway.
I decided to switch to SanDisk, and have a couple of 8GB Extreme III 30Mb/sec cards, but they always seem to fill too fast. I shoot a lot, and quickly!
Today I picked up a couple of SanDisk 16GB Ultra II 15 Mb/sec cards, and ran a little speed test.
In a nutshell, on my K20D, if you shoot JPEG (I don't), it makes almost no difference which you choose. After the buffer fills there is a difference in the timing pattern, but because of the jpeg compression time, things equal out. I stopped comparing after 15 seconds.
More interesting to me were the RAW (DNG) write times. As expected the number of shots is identical right up to about where the buffer fills. The Extreme III's get one more shot in high speed burst (16), then begin to stutter at about 2.5 seconds per shot (presumably, the actual write speed).
The 16GB Ultra II's begin to stutter one shot sooner (ony 15 shots), then take about 5 seconds per shot after that... no surprise, about 2x slower than the Extreme III card. Nice!
Ain't it nice when things work as advertised?
(And I miss the higher speed of the K10D.... I should use it more often... sigh... )
I've attached an image from some audio software of the comparison, FWIW.
The top two are RAW, the bottom two JPEG (Best quality).