More precisely, the MAXIMUM data transfer speed of the 6 Gb/s interface is twice that of the 3 Gb/s interface. However, there are NO devices now that can feed data through the interface that fast. The whole plan, here, is to employ an interface which will not limit what the connected devices can do - the limit is the device itself. There are a FEW of the new SSD "drives" that can come close to the 3 Gb/s data transfer rate, and it is anticipated that newer ones to come will exceed the older SATA 3 Gb/s interface max. So that's when the newer 6 Gb/s system will really show its superiority.
There are a FEW of the new SSD "drives" that can come close to the 3 Gb/s data transfer rate
Uhm, no? In a SATA II/3.0Gbps the maximum transfer speed is 300MBps. My old Samsung 470 SSD can hit this. I think it's rated for 280/240 read/write. The newest SSDs on the market can do 550read and some can hit around 400MB write. It won't be long before we need to get SATA speeds even faster then 6Gbps.