Honestly, it felt to me like iOS 6 was the culmination of 6 years of engineering by a proud and strong company that employs the world's best software engineers.
iOS 7 was such a huge step back in every single way. The OS just no longer feels solidly built. The design has so many inconsistencies that I find it hard to believe a world class designer like Ive designed it. It simply feels fragile. I don't remember ever having to DFU restore my iPhone to get rid of some weird bug every few months with iOS6. I don't remember worrying about "oh I did a restore instead of set as new so something will surely break."
Honestly? I miss Forstall and I think Apple really changed for the worse software-wise the day he left. These 'engineers' responsible for iOS 8 and Yosemite? Amateur hour garbage workers. I can't believe something so broken could ship from the world's richest non-oil company.
I guess it boils down to this: with iOS 6 on my old iPhone 5, I had this sense of trust in my phone. I trusted it to do the task it was assigned and it always delivered without mistake. With iOS 7 and 8, that trust is mostly gone and I'm always "eh" about whether it'll complete my task properly or not, and whatever kind of battery life it feels like having that day. The trust a person has in their phone is simply the most important element of all. I don't care about fancy blur and animations and Air this or Handoff that. Give me back a consistent, trustworthy experience.
With that out of the way, my feelings on iOS 9 are:
On one side, I remain somewhat optimistic that iOS 9 will finally be what I'm looking for. On the other side, I was similarly optimistic with iOS 7, 8, and OSX Yosemite, and they've all turned out to be nothing more than amateur hour programming and design experimentation.