Short test with an SDHC Class 10 and AJA System Test:
MB Air USB 3.0: 12 MB / s, read 22 MB / s
MB Pro USB 2.0: 12 MB / s, read 20 MB / s
For comparison, the same card in the SD slot of the MB Pro: 9 MB / s, read 21 MB / s
All this under Mac OS 10.8.
In the same MB Pro under 10.6 of the stick is not working.
Handy, simple, cheap, good.