He has strong opinions and he is really pushing haskell, he thinks it is much better than anything else. I cannot say I agree with everything he says, but he is a clever person.
I had a pleasure to meet him in person here, in Novosibirsk. Sergey organized a meetup here in 2014(15?), invited Bartosz to be a speaker. I also presented a talk that time. We were talking about functional programming in C++
Although he is very cool person, I failed to read his book on Category Theory. Too deep for me :(
It is very theoretical, abstract mathematics. Few programmers have that kind of education. Even basic abstract algebra is unknown to most people in this field, nowadays.