Is it illegal for me to use my neighbors open Wi-Fi?

My neighbor (i guess its one of my neighbors) has an open unencrypted wireless network. Its called 10FX05013721. It is a real person because i scanned the network and it has a printer and a computer connected. I have access to a their printer (i'm not using it). There internet is sooo fast compared to my wifi. Is it illegal for me to keep using it? Should i print something out telling them their network is open? should i just keep using this awesome ******* fast network?