Ventajas
I had a great supervisor, Sally, who ran a tight ship but was also a friendly and helpful mentor and didn't get angry when I made mistakes as I was starting out. I got some experience stocking the shelves, interacting with a variety of customers, and learning the value of merchandise. There's also some opportunities to get cool books, etc. for low prices, especially if no one else wants them.
Desventajas
I worked as a volunteer, since it's a charity shop, so obviously I didn't get paid. In addition, customers will sometimes have strong body odor, and like any customer service job, you just have to grin and bear it.