What do you think of the concept 'Survival of the fittest'?

"What do you think of the concept 'Survival of the fittest'?" 

The cheetah has evolved to run fast to catch prey. The prey have evolved to run fast to avoid becoming a cheetah's lunch. It's an evolutionary arms race. The fittest survive. Their genes are passed on to the next generation. 

Another example: The gradual increase in the brain size of human ancestors made it possible for them to survive in their hostile environment. 

Another example: The evolution of shells on turtles allowed them to survive attacks in the sea from above or below.