Six core processors are impractical in a gaming environment because no games use more than 3 cores. On the other hand, each core of a processor can be assigned to a different tasks. This makes them useful in home servers and individuals who do professional movie or photo work.
Yes, they are the fastest processors but generally not because they have six cores. When a new processor comes out it's 'operations per second' is usually updated as well.
Yes, the 980x is the fastest processor but it's price is set too high. Some reasons for this are that the processor is hyper-threaded which would total to 12 threads and 6 cores. Another reason for the cost is the unlocked multiplier which might help for overclocking.