From what I understand, it's not even that real. That is, if you can get all your work done in 80% of your time, then you are free to work 20% on whatever, but most people don't end up doing it because they have to spend 100% of their time (or close to it) on official work.

but, afaik, you can club all the personal time you don't use and take it as a continuous block of time, perhaps between projects. Anyone working for Google, please clarify!

I don't work there any more, but you are correct. Many successful products started with banked 20% time.

However, most individual 20% projects are not standalone projects. They are often tools or components or features or mini-sabbaticals with other teams.

It is more possible to entrepreneurial at Google than any other company I know of. The workload is really high, but even so, the main barrier to effective 20% time is still oneself. Even at Google, most people talk about their brilliant ideas and don't do anything about them.

So how is not having it any better?

So you can go home and do your own startup if you're not satisfied working with X company that doesn't give you 20%.