Who Tried Power Conditioners Over The Years and No Longer Use Them Today, Any Why?

If you are connected to the grid in anyway you need a pure sine wave AC conditioner.

Even if you you are off grid, it's highly likely you still need one.

I've been using power conditioners since my 1st Adcom ACE515 Enhancer in the early 90's and today using a Furman Elite 15PF with Monster AVS2000 stabilizer. I will never get rid of my conditioners, to important in my system and also in my opinion. 

" But profits come from satisfying consumer demand."

No Miller.  Profits come from stimulating it.

You've said it Miller:   "Selective memory nostalgia", is when you think you hear a difference after adding a passive component that you've paid a lot of money for, and then it gets locked in by the warm feeling generated by the apparently positive response to your wise spending.  We all have the urge to feel happy whether or not there is anything to be happy about.

As far as passive power cords are concerned, all will do the same job as long as they can pass all the current demanded without high resistance.

Power conditioners can be useful if they can actively remove noise and other unwanted artifacts from the line.

Interestingly,

Plugging my amp directly to the outlet (instead of three different high quality multiplugs I’d sampled) made my system more dynamic, rich, and full-sounding. It wasn’t a figment of my aural imagination…it was clear, evident, and palpable. I gave it time between configurations, and ended right back to just plugging straight in. 
Then, I replaced the cheapo outlet with a Hubbell oulet, and there was an additional improvement (although that might have attributed simply to the fixture holding the plug more firmly/increased contact). 
I still use a Furman multi for the rest of my components, for convenience. 

Many residential and commercial spaces apparently do not require power conditioning. My colleagues here in the department of electrical engineering and computer science measured the voltage in our house and within our home network. They even brought some expensive retail conditioners to test and they (and me using my ears!) - we found no measurable or significant differences. I will post their results if there is interest. There are no problems or issues here - thus, I believe that those who suggest that we can all benefit from power conditioning are wrong.