Absolutely fantastic add-on! Great for organizing many open tabs and windows. Note one thing that was not very clear early on for me is that each group is associated with only one window. So you cannot have multiple windows associated with a single group. Although I did not expect this at first, this approach gives a very fluid user experience. I cannot recommend this extension enough if you are like me and tend to keep many tabs and windows open for an absurd amount of time and return to them later.

One slight annoyance is how, from what I can tell, there is no easy way to convert a window that is not associated with a group to be a new group. If you select ALL open tabs in a window, use "Move tab to group" to a new group, it closes the window you were just on, changes the current window to another one, and changes that window to the newly created group. Expected behavior is to keep the window open when creating a new group for all tabs on the the current window.