I discovered a wonderful site devoted to Hanon: Interestingly, Sergi Rachmaninoff, in describing the 9 year piano training path for Russian pianists, says that by the fifth year each student is to have mastered Hanon's The Virtuoso Pianist
, to the point that the examiner can call out any number of an exercise and the student will play it.
Hanon exercises can be very helpful if played intelligently and musically. But if practiced mindlessly, they are not that helpful, and potentially harmful for technical development. If played in different keys, with different touches and dynamics, they are fabulous!