(Disclosure: I work for Google, but these are personal opinions.)
When iOS 8 came out, I immediately upgraded all my devices. I was unhappy with how iOS 8 requires some 5-6 GB free space to install, which meant that I had to delete all the music I’d copied to my iPhone. And the update itself took almost 20 minutes to install, which wasn’t the case with previous versions of iOS, or any version of Android. I thought that these are upgrade-specific problems, and that once the upgrade is done, I’ll have a better device than before.
I was wrong. iOS 8 makes both my iPads (iPad 2 and iPad 3) janky and unresponsive, sometimes taking multiple seconds to respond to a tap. And gestures were completely broken, since you never knew whether you performed the gesture right, and the iPad was merely working on it, or if it had rejected the gesture. I wanted to downgrade my iPad to 7, but Apple doesn’t allow downgrades.
This has never happened to me before. I’ve never installed an iOS version I regretted installing . I did regret installing an OS X update, but Apple fixed it in a few weeks. Ideally, they should have waited a few more weeks and shipped a quality release, but at least that was a temporary problem. Maybe that was an indication of worse to come, and it certainly turned out that way, with iOS 8 making my iPads unusable.
Refusing downgrades, even at the owner’s choice, is a bad policy because it assumes that Apple is perfect, which is far from the case. As far as I’m concerned, iOS 8 is a regression, not an improvement. No matter what features it does or doesn’t have, it makes my iPads unusable. After years of mocking Microsoft, Apple delivered their own version of Windows 95: slow, bloated, unresponsive. Is this progress?
I guess the next time a major version of iOS comes along (like iOS 9), I’ll install it only one of my iDevices, and try it out for a month without subjecting my other devices to it.
This is a shame, because it means that Apple has failed in shipping quality software. Why, then, will people continue to pay premium prices for shoddy, unresponsive devices? Apple needs to realise why people buy its products, not get caught up in its own marketing hype, and go back to building great devices if it wants to continue to be successful.
 I dislike iOS 7’s esthetic choices, and find it visually ugly, but it didn’t make my iPhone unusable.