The idea is good, but it has numerous issues. Half the time it doesn't work for inserting a character via enter. For example the youtube comment box. Here on AMO, the Unicode Keyboard panel doesn't even appear properly only the search field, the rest is out of screen in a tiny box, no wider than the search field.
On gmail, it doesn't do anything. Apparently gmail hijacks the hotkey for the addon.
At least an option to leave open the panel when inserting a character would be nice.