I never would have gotten a business off the ground without understanding concepts like the outside view, the introspection illusion, anchoring, fox vs hedgehog thinking, belief constraining expected evidence, basic statistics, and many more.
But it's more than that. LW type material upgraded my understanding of what it means to understand material. Previously I was doing a lot of guessing the teacher's password, CYA, moral licensing etc in justifying my actions to myself. Building models of when and where to apply concepts, how to seek out new ones when the existing concepts are insufficient, and how to validate them against problem domains is as important as the concepts themselves.
This is partly the difference between LW and just handing someone a copy of Thinking Fast and Slow. In the latter case, I would have read the book, gone "yes, that sounds very nice" and then continued on my way (I think).
I feel I've learned a lot about problem solving from less wrong (and HPMOR in particular). be concrete, hold off on proposing solutions, et cetera. The effect*, unfortunately, doesn't seem to be as much as I had hoped.
Small increases in intelligence are extremely general and even recursive, so it feels worth the effort. But, I found alternatives much more effective (though still modest) then studying/discussing/applying the sequences. like meditation or smart drugs.
I'm interested in other lesswrongers experiences in cognitive enhancement.
* by "smarter" I mean "better at problem solving", where examples of "problems" are writing a program, finding the right thing to say to resolve interpersonal conflict, memorizing some random fact quickly and then recalling it quickly/vividly. let me know if you want further clarification.