IntroYo made a huge splash when it first came out. It was all over the tech news sites and of course I had to download the app and see what the hype was. Like all the tech news sites said it was dead simple to use push a button and send a Yo. After a while I found some services via the Yo store. A Yo would be sent when a new post was made or something happened that you cared about (a package delivered, your favorite instagramer uploads a new photo, etc.).
My local ski mountain Yo's whenever 6" of new snow falls, this was pretty much the only reason I kept Yo. It was my killer app for Yo but I wanted more, enter Spark. I have a Spark Core and it is perfect for internet of things (IoT) projects. I wanted a light to turn on whenever 6" of snow fell on the moutain, so I would know even if I didn't have my phone close to me.
Yo provides a simple callback to a webpage whenever a Yo is received which is pretty awesome. The callback includes the username and location if it was included as a GET variable. The setup is fairly simple and straight forward, Yo calls back to a webpage that does some basic checks and then uses the Spark cloud to activate the light on the Spark Core.