Micro SD w/ adapter
There will be a significant drop in performance when compared to a premium SD/SDHC card but the microSD cards do perform almost as well as as a low-end SD/SDHC card when used in an adapter. They are obviously designed for different applications.
In my testing the 2GB SanDisk microSD card performed about as well as the same-capacity "standard" SanDisk (blue) SD card: About 10 MB/s read, 6 MB/s write for either card. I cannot say the same for the SDHC (4GB and larger) versions of the card because they use different IC chips than their 2GB brandmates.
Would a Class 6 micro SD compare well with a Class 6 'standard' SD, say 8GB made by SanDisk?