Have you found Coq or other formal-methods tooling useful?

No, I haven't, but I want to. I haven't had the opportunity to go deep into formal methods for a work project yet, and on personal projects I've mostly played with Ada SPARK, not Coq. I've played with Coq a little (to the extent of re-implementing the linked paper for a subset of a different ISA), but there's definitely a learning curve. One of my personal benchmark projects is to implement a (fixed-maximum-size) MMM heap in a language, and convince myself it's both correct and of high performance; and I've yet to achieve that in any language, even SPARK, without leaning heavily on testing and hand-waving along with some amount of formal proof. I've written the code and comprehensive testing in various assembly languages (again, this is sort of my benchmark bring-up-a-new-platform-in-the-brain example), and I /think/ that for the way I think proving properties as invariants maintained across individual instructions maps pretty well to how I convince myself it's correct, but I haven't had the time, depth, or toolchain to really bottom any of this out.

I have found huge value in CBMC and KLEE for statically verifying C code for real time safety critical embedded applications.

ACL2 is also VERY powerful and capable.