They're great if you do a lot of shipping.
Basically, it's a little machine that hooks up to your computer, and when you print a shipping label from Etsy (or possibly from Paypal) it prints right onto a little 4" x 6" sticker, which you then stick to your package. It's fast, is the main advantage. There is no ink to buy (it's thermal printing, which basically means magic) but of course you have to buy the stickers - there are off-brand ones on Amazon that work just fine.