Has anyone bought a class 6 sd card?

When you consider what it does it should not. The only thing that a class 6 card does is write faster. The read speed is constant. And many cheap class 6 cards do not preform as well as a good quality class 2. Since many of those class 2 cards the manufacturer was just lazy and did not want to get it tested.
 
I agree with quetzacoatl.

Its no different than upgrading the SD card in a camera or the hard drive in your computer. The camera will only function faster between taking pictures because the write speed is faster. Data access speed increases would require more on board RAM, just like with a computer.