I liked Sussman's answer. There is a huge difference in learning things deeply from the bottom up vs. 'get stuff done' approaches using lots of black box libraries, etc.

I have been using Ruby, Java, Scala, Python, and Haskell mostly for years now. But just recently I started working on the 4th edition of my old Common Lisp book and some preparatory work on a new Scheme book. I have been enjoying the Lisp-way of bottom up interactive development a lot.

Both general approaches have value. When doing machine learning it is great to have the universe of useful libraries for Python but for green field work that is more algorithmic, doing bottom up development in languages like Scheme or Haskell is also great.