I ran it on my project, I made some PRs that let it compile and give some performance improvements, which have now been merged. I still use cargo to validate tests, but it's been extremely useful for quickly testing the app before accepting changes.

So far the test suite runs very slowly compared to cargo, and I haven't tested the hot patching. But that's fine; all that is automated, so Relax greatly speeds up the part that I'm actually needed for.