As I purchased a USB3 card reader this week, decided to test it out for the first time this evening.
What I copied was 160 Nikon NEF files totaling 2.13GB of data.
* I used my old USB2 card reader plugged into a USB2 port on my computer
* I used my new Sandisk USB3 card reader plugged into a USB3 port on my computer
I copied the same 160 NEF files off the cards, to the same directory on my HDD on my computer.
The USB2 card reader copied the files in 2 minutes 49 seconds
The USB3 card reader copied the files in 2 minutes 5 seconds.
I found these results rather interesting, in that USB3 specs should be much faster than USB2, but real-world testing (at least for me) shows that whilst faster, the transfer speed advantage is not as significant as I expected.