It's card stock! What can be said about plain white card stock? Staples brand was the cheapest. It runs through our Copystar CS6551ci perfectly. It will double-side if you use the 'cardstock' media setting for the printer and feed it from the MP tray.
I bought this to make holiday cards and birthday invitations/thank you's and it works great. Our copier prints beautifully on these. I have used them with a laser printer and an inkjet printer and they hold up well. They're also nice and bright in addition to thick. Totally adorbz for any use.