So I recently read this post on Etsy Marketing about Promo Packs but also this thread about them on the Etsy forums. People definitely seem divided on whether they're a good idea. With this controversy over promo packs you should really think about whether or not it fits your business. Also keep in mind that if your shipping very light or thin items a promo pack may increase your shipping costs some depending on the type of shipping your using.
Personally I like the idea, it's a simple little extra to include in your orders, cheap marketing, and I like getting free stuff. However keep in mind that your packs should have balance, I don't think anyone likes just getting a bunch of business cards in a zip bag. So If you decide to do a promo pack swap make sure you are getting not just business cards but also samples or other freebies like stickers, bookmarks, etc.
I've said it before and I'm going to keep repeating it: packaging matters. Make your promo packs look like a gift. One idea would be to get some little cello bags or colored cellophane (the crisp kind for gifts not the soft stuff for saving leftovers) and tie it off with a little ribbon and a tag.