For me act works fairly well, though it isn't exactly the same as Github. Matrix builds didn't work properly (just noticed it has been fixed now), and the base images aren't quite the same.

Github should send a bunch of money to the act developer - I know I wouldn't have used Github actions at all without act existing, I'm sure other people must be in the same situation. (Though I'm not paying Github either, so perhaps I'm not a target customer...)

Like many things, the more complex the workflow the more useful it becomes running it locally.

I've found it especially useful for fixing complex workflows or working with custom actions. It's not strictly needed, but it does speed up your workflow once you figure out the kinks.