Has anyone any experience of using heml.io?

They claim "HEML is an open source markup language for building responsive email. It gives you the native power of HTML without having to deal with all of the email quirks. HEML makes building emails as easy as building websites."

Someone has just reccommended it to me and i'm planning on having a look at it but wanted to see if anyone else had used and had any positives/negatives about it?