More years ago than I care to remember, I got the Hanon exercises of my own volition and went through them quite diligently for a while largely with the aim of trying to ascertain whether they'd help me to figure out why I hadn't the slightest natural ability to play the instrument in spite of my abiding interest in it. They didn't. They did nothing to help my "facility" (or rather lack thereof) either. One problem that I found with them is that they all require the use of both thumbs and all eight fingers and so there are no digits left to put on one's ears while working at them.
I can see what they're supposedly for, obviously, but spending more than a few minutes at them on a non-daily basis might risk dulling one's musical sensibilities or even, in extreme cases, putting one off playing altogether. I have often said that the pianist who can perform really well the études of Chopin, Liszt and Alkan as well as the Chopin/Godowsky ones is equipped to tackle almost anything in the piano repertoire, however challenging it may be. OK, a whole lot of that - especially the last named - will be beyond the wit and fingers of most pianists under the age of 14 but, for them, what's so wrong or so comparatively limited in use with practising scales and arpeggios - not just by rote or always one octave apart as in the books of them but more inventively - bitonal scales and arpeggios, or three octaves apart, or in all keys but using the standard fingering for C major - and not neglecting some single and double note scales in thirds, fourths &c.? - provided that undue amounts of time are not spent on too much this - and then there's also the WTC, whose value in developing keyboard skills should never be underestimated!
Or maybe the best thing to do with Hanon is what's happend on this thread; leave it alone for around five years, return to thinking about it for abit and then abandon them for another five years or so...
Best,
Alistair