Church was the original probabilistic programming language used for the book, and as per another comment in this thread and a draft PhD dissertation[1], WebPPL is the successor to Church.

"Niche" is probably in the eyes of the beholder if the audience is deliberately diving into probabilistic programming languages and probabilistic cognitive models.

[1] http://library.meritology.com/fundamentals/chapters/1a-In_a_...

It's basically scheme with built in probabilistic functions.

If you know scheme or lisp it should be no different than learning to use a few new functions.

If you are not familiar with functional programming you might find it difficult to learn.

I know I found it difficult.

You can always use PyMC or some other probabilistic programming library/language.