I did this for maybe a year and a half while getting my mind around the idea of reading professionally. It was a fabulous experience for me--I learned so much about each individual card, what my own take on each card was, how to phrase a reading, etc.
The way it was organized made it very simple, in that I was able to set the number of readings I wanted to do on a given day, plus you can give away a reading if something comes up and you are unable to do it, or pick up an extra reading that another reader couldn't do.
The mentor I was initially assigned to barely communicated, and was really no use at all, but the second mentor I received was a professional reader who was very encouraging and helpful to me. Also, I received feedback from about a third of the readings, which was of great benefit to me, as well.
I think I did around 600+ readings for the ATA, and it afforded me a step I personally needed between having learned the cards and being able to read comfortably for strangers. It also gave me a really good idea of what types of questions to expect. The experience gave me the confidence to proceed to the next step of reading professionally at a local shop.
