The one feature that really helps it stand out from others is that you have the ability to annotate Google Docs in many ways, such as leaving comments or even drawing on the document. The annotations are only found when logged into the Chalkup site and you will not be able to see them on the original Google Doc, only the copy in the assignment submission. There are a lot of people who want this feature and this is one of the few easy ways I have found to be able to do this.
(sidenote: There are a lot of ways that we can leave effective feedback for students, we should look for new ways that allow us to leave better feedback that helps our students as learners so make sure that any feedback you give to students is effective, not just quick.)
Chalkup is easy to setup and join, thanks to using your Google account, so you can easily create a class and start trying it out. Watch the video below to see how to get started. Richard Byrne has also written some great posts about Chalkup, including about their rubric features, at http://www.freetech4teachers.com/2015/02/chalkup-grade-assignments-in-google.html#.VRQB6zvF9g4.