Excellent for reading or general browsing, the popup is perfect.

However, it's very annoying when editing. The extension needs: (A) a toolbar icon to quickly enable/disable the popup. (B) an option to disable the popup on selections inside inputs or textareas. (C) perhaps whitelists and blacklists of pages/domains where the popup will/won't be used.

Also, it's somewhat concerning that this is not open-source -- for more easily vetting the safety and for submitting pull-requests.