When everyone signs it, the host has a reminder of the fun times. Especially when they write something sarcastic. I tell them that it's like signing a yearbook, which usually gets everyone laughing.
My standard note, that I do customize based on how well I know the host, goes something like: Name, Thanks for opening your home to me and your friends. I just know that you'll love all your new tools. Enjoy these recipes! Ann
I usually sign it first so that people can see that they can write a note, and not just their names.