Whether it is useful or not depends on the contents of your spam folder and already one person in this thread reports that for them it certainly was useful.
I'm not sitting here making stuff up and I seriously resent your suggestion that this is false.
There's zero evidence to show that, "Gmail not to be trusted with important email.", as the title suggests. None of it is factual. All of it is presented as-is with only what appears to be a very poor understanding of MTA configuration issues.
The answer has been here all along early in the thread. DKIM & SPF yet the complainant has simply shrugged it off with, "Well that would cause consistent issues." (which is also false.)
If he can not do basic troubleshooting or understand the importance of the SPF/DKIM settings in a modern MTA system, and as a basic starting point, he can not assert that Google can not be trusted with important emails.
So. False, and useless.
1. some Gmail users have told me they don't receive email from my non-gmail server/address, even though I set up SPF, reverse DNS lookup and everything else that was recommended except for DKIM. (The domain was bottoms.christianjaeger.ch.)
2. I've gotten a lot of false positives from mailing lists in my Gmail spam folder for a long time. Now the part that really pisses me off is that Gmail's learning mechanism, if present at all, does not seem to work at all. Why otherwise would mails from the same mailing lists, the same authors, with the same kind of content (e.g. "[PATCH]" in the subject, patch in the mail body) still make it to the spam folder with the noted reason "looks similar to other mails treated as spam"?
3. On top of that, every about 10 mails I mark as spam the Gmail UI asks me whether I'd like to submit it to the Gmail spam team to help improve the service. I never got any notice from them, nor does this seem to have helped.
4. I think I might be spending more time now going over my spam folder and unspam false positives there than I was going over my inbox in the past and tag the false negatives when I was hosting my own mail and used spamsassassin and grey listing.
5. It's getting worse and worse. Now all messages from my servers (crontab and other system mail) are going to spam, too[1]. So basically I now have to move to something else, and I'm ready to use my own hosting again, but I don't know what I should do to solve the "Gmail users won't receive my mail anymore" problem.
[1] e.g. From address "[email protected]" with "Return-Path: <[email protected]>", or From "[email protected]" with that same address as Return-Path. (Also a header from Google says "Received-SPF: pass (google.com: domain of [email protected] designates 78.47.51.206 as permitted sender)" or "Received-SPF: pass (google.com: domain of [email protected] designates 78.47.51.206 as permitted sender)" for the latter.)
Edit: add examples