Even worse, I've become accustomed specifically to vertical 'tree-style' tabs. I hate that I need them - it makes me less flexible than I would want to be.
The simple fact it is a separate window bothers me. I know you can't change the Chrome UI without using C++ but the Firefox version looks and works much better.
Until Chrome lets people modify the browser UI like Tree Style Tab and other addons do, I won't be interested in switching. It's a good browser for my parents, computer labs, etc. but I could never use it on my workstation.
The other simpler addon is not a separate window, but this one is much better once you get used to it. Worth it as it really takes things to a different level.
BTW you can move the tab outliner itself to a tab and close the window.
If you haven't used it before, and if you're prone to having 10+ tabs open at a time, I highly recommend it. Although I warn you that you may never wish to use Safari or Chrome again...
Chrome used to have a similar feature built-in but they removed it ages ago.