Hi,
Amateur pianist here. I started young, not three or four years of age, but young enough that I can only vaguely remember a time when I could not read music but yet could read words. I might have been eight or nine when I started and I'm now in my twenties. I cannot make broad, sweeping generalisations about what is good but I can share my experience.
I had a music teacher in my formative years and took ABRSM exams up to grade 3 or 4.
This lady taught me all the foundations of music and when movement necessitated the end of lessons I never took any further lessons.
Now, she was not big on technical exercises but would encourage the isolation of difficult sections as well as hands seperate practice of difficult passages and scales/arps.
She would also enourage slow practice if necessary often forcing one to slow down saying, "Speed will come."
Apart from explaining the need to pass the thumb in scales, and general posture etc, she never gave detailed technical advice.
She would listen and say when it's not right and you had to figure out how to get it right.
For example, it is much later that I consciously read about 'rotation' in Alberti Bass and such figures despite the fact that I naturally adopted the motion to facilitate rapid, comfortable, and musical execution in mozart sonatas.
Apart from the need to play scales and arpeggios (which I hated), I was alien to the idea of 'banging out' technical exercies.
Eventually someone gave her a copy of Hannon and she introduced me to a few exercises.
She really did not put much emphasis on it apart from maybe opening the lesson with a page. Eventually, I copied her book.
Fast forward a few years later.
I've moved and just got back a piano.
I'm horrified at how sloppy my playing is.
So, teacherless, I begin to devise a plan to regain my technique.
Now, by this time I had the internet and access to a wealth of opinions.
Yes, there were people extolling the virtues of Hannon and saying one needed to play from the fingertips.
This was an exploratory period for me and I am an open minded person so I tried it.
What was the result?
At first it was OK, I felt a little evenness coming back.
But I was not palying Hannon alone so I'm not sure what to attribute that to.
What I do know is that I got tired of them quickly.
The Hannon exercies did not motivate me at all; page after dull page that I didn't even have to read more than a single bar of.
Sometimes after workign through a few pages of Hannon in the recommended manner, I did not feel like practicing anything else.
Furthermore, after a few days the curled finger position had caused the flesh to separate from under the nail of my right index finger and I seemed to have developed some kind of wrist cramp.
For the first time in my life I felt physical pain playing the piano.
Since both problems would only be exacerbated by further practice I took some days off and refected on the fruits of my efforts.
The conclusion was not difficult.
I scrapped all the advice that I was reading and went back to being a 'natural' player; the one who did what sounded and felt good NOW.
It is at this point that I started developing rather strong feelings that many of the common recommendations are simply incorrect in the general case.
Afterward, I came upon writings of Chopin, Sandor, Chang, and later Bernhard that seemed to validate what I was thinking and the methods by which I was trained.
So what is my personal albeit insiginificant verdict on Hanon?
IMO, the only potential benefit of Hanon is for warm up in the sense of getting the playing muscles physically ready.
However, scales or a Bach invention will do the same with more technical benefit in the former case and with the addition of musical meaning in the latter case.
I don't understand the acquisition of technique without music; technique exists in the service of music.
Yes, there may be some standard physical movements that have been devised to facilitate the execution of certain musical passages but they are the means not the end.
The end is to achieve that ideal ringing inside your minds ear and a total unification of your playing mechanism with your will and creative muse.
Insofar as Hannon and most technical exercies encourage separation of the mind's musical ideal from the fingertips (or render a mind-image non-existent), they are bad.
I think the relative insensitivity of the piano and the formidable coordination challenges playing it presents makes piano players more likely to focus on nebulous extra-musical technique.
I can't imagine a violinist practicing technique separately from musicality.
It is impossible since they always have to be concerned about being in tune so they have to *listen* and *adjust*; there is constant feedback from ears through mind to muscles.
There is no choosing between one and the other; producing a clean musical tone is PART of the technique of playing the violin.
However, we on the piano have a nice pre-packaged tone and don't even have to tune our instruments ourselves so we sometimes imagine we can treat it like a typewriter.
I also think it's very telling that many of the great pianists of the past were composers and excellent improvisors as well.
I never make a mistake when I'm playing around at the piano, even when I'm doing things that would ordinarily be difficult.
Why can't ordinary playing be as effortless, unifying and self-fulfilling?
I don't think Hannon, pursued as a habit, helps achieve this but rather encourages the opposite through learned preoccupation with the mechanism only, to the detriment of the mind and ears.
[I ended up writing rather much because I thought a lot about this in the process of guiding my own progress. If you read it all, bless you. If not, I don't carry a grudge.

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All of the above with a Mega-IMHO, of course.