I do use ACL2, although I don’t do many proofs. When I do, the proofs usually go through automatically or require me to state only a few lemmas or tell the tool how to do the induction.
This is partially a commentary on how good the automation is in ACL2 and partially a commentary on the fact that I don’t use it for hard problems :)
I use it more for property based testing. The counterexample generator is very powerful because it leverages the knowledge base of the theorem prover (all the accumulated lemmas, etc.) among other things.
