I did a few years ago and was pleased with the charm squares I received. It was nice to see the different fabric lines "in person" and not as a picture on the computer. I stopped because I was kinda overrun with charm squares and never seemed to get around to using all of them. In other words, my stash was growing faster than my ability to sew.
I did the 2 yr program and it seemed to work out to about $5.00 per pkg. And 20 charms is about half of a regular pack, not sure what the cost of a charm pack is now as I quit buying charms, fat quarters, well any fabric really.
If you are into scrappy, it is a perfect way to get a lot of variety.