This is a speed test of two cards, both tested with USB 3.0 card reader on USB 3.0 port, running Windows 8
Left: Sandisk Ultra Class 6 16GB, bought at Sandisk booth on IT show. Rated speed: 30MB/s read, write speed lower
Right: Kingston Class 4 8GB, free gift from DSLR purchase. Rated speed: Unknown
From the graph, we can see that Sandisk's read speed is even higher than rated speed, but the write speed is even less than 8MB/s... it turns out that the "write speed lower" means about 4 times slower.... even a free gift kingston class 4 writes faster about 10MB/s
And when use in DSLR, it is the write speed that matters... that's why I don't recommend Sandisk Ultra series.