Unfortunately it's not fully-fledged enough to work properly with all CI configs. I had trouble with some conditions but also I note that it doesn't seem to implement reusable workflows yet. It's also a bit fiddly to set up so you don't run into API rate limits.

I think that people who are interested in having resilience should write their GHA CI config for act and then be happily surprised when it works on GitHub too. Or write them as something more portable (Earthfiles are interesting, but a bit clunky when I tried) that can also be executed locally — having the GitHub-specific stuff just be a very light wrapper to 'call into' the portable scripts.