I'm a computer science student. I did a course on information theory, and I'm currently doing a course on Universal AI (taught by Marcus Hutter himself!). I've found both of these courses far easier as a result of already having a strong intuition for the topics, thanks to seeing them discussed on LW in a qualitative way.

For example, Bayes' theorem, Shannon entropy, Kolmogorov complexity, sequential decision theory, and AIXI are all topics which I feel I've understood far better thanks to reading LW.

LW also inspired me to read a lot of philosophy. AFAICT, I know about as much philosophy as a second or third year philosophy student at my university, and I'm better at thinking about it than most of them are, thanks to the fantastic experience of reading and participating in discussion here. So that counts as useful.