I found SICP was better for learning to "think functionally". PCL and On Lisp were both great in their own right, but idiomatic CL isn't as strictly functional

I found SICP was better for learning to "think functionally". PCL and On Lisp were both great in their own right, but idiomatic CL isn't as strictly functional as idiomatic Scheme tends to be (and neither enforces it like Haskell and Erlang do). As for improving my skills, that's an unequivocal yes. I think the biggest eureka moment came when I understood the idea of first class functions. That opened many doors to better code. Before, I used functions primarily like a writer uses paragraphs: to break up the narrative into chunks. After SICP, I saw functions more like (for lack of a better term) filters. They transform their input into output. The transformation is well defined, and the natural boundaries between functions become clear rather than arbitrary.