Rated 4 out of 5 stars

Very nice, but the indicators are totally counter-intuitive IMO. Google got it wrong, and you shouldn't feel the need to copy them. Perhaps > for personal; >> for multi; >>> for mailing list; or reduce each by one > ie nothing for personal

This review is for a previous version of the add-on (0.5). 

Henry, thanks for the review. I agree that the indicators can be seen counter-intuitive when not used to Google Mail. I use both, thus I of course feel the need to copy them, just to have it in sync. Anyways, I'll see if I can make it configurable.

Edit: Since version 0.7 the indicators are configurable and provide a second style (triple style), which implements your suggestion.