Grockit is very easy to use and has a user friendly interface. It lets its users search for a YouTube video and once found, they can then both watch the video and use the right side panel to write their questions or comments. They can then share this page using a generated URL with their students or colleagues.
Grockit allows for greater interaction around youtube videos. Students can use it to review video materials and add their questions or time them to specific points in the video. I think that teachers need to try this tool and see by themselves how practically useful it is.
Check This Example to see what Grockit can do. The example is provided by Nik , you can also read his detailed post about Grockit in Nik' Quick Shout Blog.