I've been at Google for 3 years and have 20%ed the entire time I've been there on: grpc-go, Drive, and Go tools (gopls, etc).
I think it's fantastic. The whole 120% thing is up to the individual: there have been times I've made it a 120%, and there are times when it's been just "take a friday off to work on other stuff". You end up getting less of your "job" done but my managers have always been supportive.
It's been great for sanity: some weeks/months feel just, like, meetings and chore work. It's great having that one day a week to work on a rockstar feature request in some fun project. It's also cool to work on your dream projects without the luck/physical move/whatever to get on the actual team. (you can effectively work on anything since no project is going to say no to free headcount)