I don't have to get approval to take 20% time, and I work with a number of people on their 20% projects.
I can also confirm that many people don't take their 20% time. Whether it's culture change due to new hiring, lack of imagination, pressure to excel on their primary project, I'm not sure, but it is disappointing. Still, in engineering No permission is needed.
There is a reason the article talked to many ex-googlers because they are ones who, perhaps, left when there was a difference of opinion about their contribution to the company. And if that conversation attempted to use some of their 20% projects as a way of contributing to their contribution, and that was the disagreement, well it ends up with them leaving.
I left in 2010, which was the start of the lurch toward more managers, and those managers were being scored on what their team accomplished that was assigned to the manager, not on what their team accomplished on their 20% time. Some managers had taken that to mean that 20% time didn't count at all toward calibration, and if you were working only 80% of the time on their projects it counted against calibration. Hence the disconnect. That is why "120%" sort of works, even in the face of a manager trying to make their own number. Except the benefit sounds different if you say "And on Saturday you can work on any project you want." :-)
The policy of course is to not do that, but how would Google enforce that? If you spent 200% of your time on your primary project, you might get excellent reviews. Should you be calibrated 50% lower to normalize everyone? So yes, if you work 100% on your primary and +20% on your 20p, then you might get higher perf scores. Eh, if you have a good idea how to correct for that, go ahead, but it's a people problem and people are hard to deal with in these kinds of thing.
This is one reason why I actively encourage as many people to take their 20% time as possible. :)
My personal experience though is very positive regarding 20% time. My first project, I started, gathered a few contributors, built a prototype, pitched it and turned it into a full-fledged project. I got great reviews and promoted in large part because of that. In my second PA I work on open source projects. Less glamorous, and maybe less perf-impacting, but that's not my goal right now.
In general the people I've seen take 20% time in a focused way tend to be the higher-performers. They're more self-guided, more critical of problems that need to be solved, or use 20% time to teach themselves or move onto harder problems. Maybe as we've grown the percentage of employees like that has shrunk, maybe it takes a while to realize you have 20% time. Not sure, but I'd like to see it be used more, it's good for everyone.
I'm not being critical of your account, I'm glad it has worked for you, as the manager of the engineering effort here I want to take the "good" stuff and use it, and leave behind the "not so good" stuff. So that is my agenda in this discussion, nothing more, nothing less.
"In general the people I've seen take 20% time in a focused way tend to be the higher-performers."
That is my experience as well, and regardless of a company policy for or against 20% time, focussed, high performing engineers will spend 20% of their time do cool and innovative things. Whether they do it at home or at work doesn't really matter, and having them do it at work is good for the company.
So the interesting thing about the policy is to capture the next tier of engineers and help them to be more productive by encouraging them to develop habits of the aforementioned highly successful focussed engineers. And to weed out the folks who are abusing the program  or at least not being any more productive with it.
If nothing else people are different right? Facebook's response has been "hackathons" which carries with it some characteristics of high performers (who quickly prototype ideas to test their validity or get a handle on their challenges)
But in all those scenarios, if you have managers, you need to also train your management on what the program is trying to achieve and how it might be addressed. So you don't end up with some managers giving their people 1 day a week off, and some demanding they work on Saturday if they want to use that extra time.
If it is "You have this huge resource available, dare to use it." then you can manage to that without damaging either morale or perf scores. From the anecdotes in the OP article it sounds like they are still working on that part.
 Like the guy who said he was trying to capture the great ideas he dreamed about in his 20% time so he would spend several hours napping for an hour and then waking up and writing down what he dreamed about.
My previous experience is mostly in R&D startups, doing robotics and natural language processing. I was hired in 2010 and then found out I would be working on YouTube ads. I was disappointed, but decided to try to make the best of it. After three months I realized my lack of interest was going to be a problem, so I talked to my manager about trying to transfer. He discussed it with the site director, and the response was "we don't care" and I couldn't transfer until I'd been there 18 months.
I decided to stick around and see if I could work the system in some way, with the probably naive thought of trying to demonstrate my abilities and catch someone's attention that would help me transfer to a project I'd enjoy where I could make a real contribution.
During Innovation Week (a hackathon) I led a team of three other engineers working on an idea I came up with, and we won the "Most Innovative" award. The other engineers decided they all wanted to devote their 20% time to working on my idea.
My Tech Lead and my manager had no interest in my Innovation Week project, and I still had no way out of YouTube ads. Unsurprisingly my performance on my 100% project wasn't great and even if I made it to 18 months it seemed unlikely that I'd be able to transfer. I left after 17 months at Google.
I've mentioned this story before, and I hope I'm not just grinding an axe--I'm just telling my experience in the hope that it will inform engineers about possible outcomes of working for Google. I am fully responsible for my experience there, but I can say the priorities of the (large, heterogeneous) company were not what I (again, probably naively) expected.
FYI, when it comes to reading any Michael Church claim, especially one about working at Google, I'd put more weight on the "outspoken" than the "ex-Googler." He admitted himself on HN that he engaged in what he called "white hat trolling" while at Google, a pattern he seems to have repeated throughout his life.
He's very articulate, but he has an EXTREMELY strong belief in the quality and accuracy of his own ideas. For illustration, he wrote a few months ago, "societies live or die based on what proportion of the few thousand people like me per generation get their ideas into implementation." When presented with such self-confidence beyond all reasonable proportion and corroborating evidence, I think an appropriate response is skepticism.
Engineers get 20% time period. You can be asked to defer it for a quarter. On the other hand, most don't take it.
I don't know him personally but I read the stuff he writes, and while his character can be a bit abrasive he's an extremely intelligent dude who can make astute observations and connections that other people miss. I think you're doing him a lot of disservice by dismissing him the way you did.
(That's a trick question, since the answer is going to depend on the person's intelligence and ability to make observations.)
Also, a ton of long-time Googlers agree with the stuff he says. It's not like he's some lone dissenting voice.
Personally, I did have very soft discouragement against wide-open 20% time. I asked around and a lot of people I talked to initially advised me against starting a 20% project so early, and especially against starting a new project rather than working on an existing one with engineers at a higher level than me. At least, they said, make sure I could get reviews out of it. That's not policy, but advice. People have their own theories about how best to get noticed and get promoted. Some of that has to do with 20% time. I guess if you're solely interested in promotion then you give more weight to such advice. I hope most Goolger's aren't solely interested in promotion.
I'm very glad I ignored that advice, both because I got to do very interesting things and because I got recognized for it, and I'm glad that I could ignore the advice because of our policy.
Or are you in an unintendedly unique position?
That doesn't seem to track with the article, or the articles/blogs linked from the article.
Lots of no-comments from the PTBs at Google... too bad. They could end the discussion one way or the other quickly.
I can't imagine my former TLM ever taking 20% time, but I can thoroughly imagine him encouraging every single one of his direct reports to do so, unless we were on a launch sprint.
I don't want to equate this with working at Google, because I don't have enough information, but here's my anecdote about good managers:
Twice before I've worked with spectacular managers. They treated their employees well, and acted as great shields against the political infighting inside the company. But both times, the managers were forced out, and the employees that depended on them to get interesting work done ended up quitting as well.
If this is indeed, as the TFA posits, a memo from the top being filtered out by a few good managers, in a short amount of time those managers won't matter; they will be forced, or burnt, out. Neither case is good for the company.
If you need humans instead of robots doing a job, that usually (increasingly, as automation advances) means it requires judgement such that pre-written inflexible rules will be inadequate to handle it sufficiently. Which means you need to rely on the judgement of people applying flexible rules for the best results.
Rather, I've always imagined that it's not that it'd be closer to the perks like "unlimited vacation time": you can't just decide "well I'm taking the next 5 years off", because that'd be a dereliction of duty. In other words, if you have the premier release of your main project coming up in 2 weeks, it may be in your best interest to not take that 20% time for the time-being, but instead use that time to fully ensure that your project has a successful launch. But, if you don't have any big deadlines coming up soon, and you're reasonably on schedule, then by all means take the 20% time.
The 20% time is a perk, and just like many other perks, your use of the perk cannot preclude any tacit duties that you may have, which include delivering on your assigned projects.
But, this is just an outsider's view, I could be completely wrong.