I like it. It makes sense.
topoftree.jp/en/tree/
Yes. The difference the column structure means is incredible. I knew I had to have it when I saw the first screenshot, and I was not disappointed; it’s a totally new “brainstorming feeling” in comparison with traditional outliners (like OmniOutline, for example). I recommend it.
Two others to explore and compare:
Ginko gingkoapp.com/
LitLift litlift.info/
Let’s not forget that a spreadsheet
has both rows & columns
for outlining purposes.
I’ve used Tree and I find it interesting. I think it is better for highly structured documents with a timeline- novel, plays etc… , where it is good to have structure and content visible. How are you using it differently than a regular outliner ? What types of documents are you outlining ? How does outlining horizontally change how you process your thoughts and ideas ?
Tinderbox has a view like this as well, called “Chart view”. For myself, I’ve found views like this to be useful when there are a lot of small headers and not a lot of child density per-node. It starts to break down a bit with long titles and excessive child counts will waste a lot of screen space. One thing the model has going for it is taking advantage of the modern fetish with widescreen monitors. 
Thanks for pointing me at Ginkoapp. It’s a similar concept to Tree but more suited to lots of text (& pictures) so good for ‘chunking’ large blocks of structured text. I’ve tried using spreadsheets but a lot of fiddling and merging of cells is required.
Gingko looks nice; I would buy it if it were an application. But I’m too paranoid to use online word processors. Not to mention that 1 month Gingko costs about as much as Tree for lifetime.
LitLift.info has a lot, but no info about the application at all … 
Here’s an in-depth review of Tree by Steve Zeoli - from three years ago, but still applicable, I think.
My sentiment exactly.
ps
Tree has now been updated to Tree2, and with a big price discount ($8US) for another couple of days, I think I’m gonna try it.
Unfortunately, because the App Store won’t allow demo versions, I had to try writing a story with the old version available on the website, and I found it pretty cool for shorter pieces where maybe Scrivener would be overkill. And I do agree with the comment above that there’s something about horizontal view that makes me see the story a little differently.
The only question I have is whether it’s spending even that amount if all I’m going to achieve is what I can do already so easily (thanks to years of practice) with Scrivener. Beyond the horizontal view, the other difference is that you can use it as a plain text/Markdown editor, by following this helpful advice. The Tree site also pointed me to a video tutorial by a guy who uses it in conjunction with Scrivener. Anyway, just curious to know how Tree users (either v. 1 or v.2) are liking it and how it complements Scrivener.
Thanks for the signpost, brett.