Must-have add-on. Rated 5 out of 5 stars
Firstly, it's worth pointing out that tabs are better on the side than the top, if you have a widescreen monitor (or anything above 1024px wide). In a browser, extra horizontal space is usually wasted, but extra vertical space is always good. Having tabs on the side also means that you can see more tabs at once, and keep them at a readable width. It is also quicker and easier to read them stacked, rather than side-by-side.
Now, for the tree style: it's brilliant. It does exactly what it says on the tin; it works very smoothly and has a consistent design. But where the tree-style really shines is where you have a lot of tabs open.
Let's say you've done a web search and opened 5 results, you have 10 photos from a flickr album, and you have a never-ending wikipedia clicktrail. Even before you do anything, the layout makes these 3 distinct groups visually obvious. But suppose you decide to look at those pictures later? Since they're all in the same group, you just click the arrow to collapse them, and you get all the space back, without losing the tabs or using multiple windows.
In short, this add-on is part of a perfect browsing experience. I've used it for a few days and I know I'm never going back.