Hi Léonie:
I was quite excited to find this feature on GarageBand, but when I used it I found that the sound of the piano (this piece is just one midi piano) was distorted so I'm going to still resort to re-entering most of the piece. Do you think my problem is that my 7 year old iMac is too old and just not with enough memory of what ever sort it uses for this type of work?
Just as an aside, I have been using GarageBand for about 6 years to create all of my choral accompaniments to the choirs that perform at school where I teach as music teacher. I'm now in the process of moving up to Logic Pro - a long term learning process - primarily for the ability to start creating my own music. I'm surely due for a more powerful iMac with this in mind and will probably be replacing my iMac this or next year.
Regardless, I'm hoping that it's my iMac's lack of power that is causing the GarageBand "follow pitch & tempo" to cause resulting distortion because I'd rather think that the GarageBand option works well if afforded proper hardware.