Looks interesting. I have tried to use a few annotation tools before within the team but people were more comfortable and accustomed to emails and we ended up switching back.
This looks like a perfect solution
I have been using this tool for a while. A prefect collaboration tool for Gmail Attachments.
Thanks a lot! I'm the founder and would love to hear about how you like it. We're constantly improving the extension and thinking about how we could add value to the much abused email attachments. :-)