First, in my youth I played a few Czerny etudes and found them not particularly helpful, as there is little transfer from them to the Chopin, Debussy or Liszt etudes which are true concert etudes. Time would be better spent on the latter, not the former. So I do not believe there is much value in Czerny.
As for Hanon, in my time I've played all of Parts I and II and have used Part III selectively based on specific needs. The scale fingerings there are definitely useful. Other than that, I reserve Hanon for one purpose only: Once in awhile we pianists have a day where our playing seems a bit uneven or ragged (probably attributable as biorhythms being out of synch.) I find that playing through Part II at MM = 100 remedies that problem nicely and takes only a few minutes. Other than that, I've found no other benefit(s) in Hanon. Rachmaninoff used to deal with the unevenness problem by stopping what he was doing and turning to Scarlatti, which is probably the better idea.
One thing that as pianists we must be able to play at all times are scales and arpeggios in parallel, four octaves, in both major and harmonic minor (more complex than melodic minor) modes. The practical reason for this is that scalar and arpeggiated figurations very frequently appear in the piano literature. By knowing the scales and arpeggios, most often the very same fingerings can be applied in musical contexts. Thus, there is real value there.
Overall, I'm a strong believer that pianists do not develop technique through "exercise books" such as Hanon, Pishna, Philipp, Cramer, Schmidt, Czerny or the others. Rather, technique is developed within the repertoire when during practicing we encounter technical problems. By isolating each problem with a bit of context to either side of it, we can make a real-world exercise out of the difficult figuration. In successfully solving it, we have enhanced our technique, and that skill is then transferable to similar passages in other works that we might face. In this approach, precious practice time is not squandered on mindless exercises, but more profitably spent on repertoire, and therein enhancing piano technique as we progress.
P.S. I just noticed that I posted this which is so similar to the posting above a year later. I must have been really tired. Anyway, I'm consistent!!!