I have had this question for a while. I saw the MIT opencourseware lectures on Cilk++.
The interface for using threads looked straight forward and the tooling seemed useful. The lecturer explained the pros of using Cilk++ instead of pthread or OpenMP. Through general bookstore browsing I have come across a book on Cilk++ once.
But inspite of all the good things that MIT would say about Cilk++, which is now owned by Intel, it seems like there is little adoption of it.
Evidence of this, however small or skewed it could be, would be the lack of published books and the number or followers or tagged questions on SO. (at the time of writing)
Amazon search gives
1. Cilk++ 2 results
2. pthreads 166 results
3. OpenMP 278 results
SO tags
1. Cilk 11 followers
2. OpenMP 242 followers
3. pthreads 258 followers
What would be the possible reasons for the slow/little adoption of Cilk++?