I love doing block exchanges, round/row robins, and some times secret sister exchanges. I will say that I have been much happier when it is a group where everyone knows each other pretty well and is at least at an advanced beginner level. There should be clearly defined rules such as the quality of the fabrics required, size of the blocks, theme if applicable, certain colors if required, whether or not the fabric has to be pre-washed, to name just a few. If there is a hostess who will receive everyone's blocks and then swap them out, she should be willing to address the issue of poorly made blocks or ones that don't follow the other rules. Quite often hostesses worry so much about not hurting the feelings of the people who don't follow the rules that they end up hurting the feelings of the people who do. That is really unfair and groups will end up losing participants.
As to some of you who never received back quilt tops, blocks, etc. contact the U. S. Postal General's office. Your local postmaster can give you the information on how to do this. It is illegal to keep another person's property and they can be charged with mail fraud. That would be the extreme, but it has been known to happen. Generally just a letter from the PO General will get them to at least return the item. I have heard of circumstances where the recipient passed away & her spouse was not clear what to do with several boxes that had come in the mail. Why he didn't just mark them return to sender is beyond me, but he did return everyone's tops after receiving a letter from the USPS.