This is a great product. Worthy of a detailed review, so here goes...
It's weakness lies in its look. It's still stuck in the OS 9 days. It is about time they updated the interface to reflect modern OS X features. But don't think it's not being developed - it is still very much an alive project and the developers are quick to respond to support issues and questions.
However, looks aside, its interface and functionality are top-notch, and you can take it as far as you want to - keep it simple, or really drill-down to the meat of the package.
I tend to first create a load of plot points that sketch out the story, then arrange the pace of the story roughly into a basic 3-act structure, then flesh out from there with more plot points, chapters, etc.
You can even enter details about the "ticking clock" of a plot point, what plot points must be achieved in particular chapter/act, how this bit moves the story forward, the height of the tension, and a lot more, and it can present all this information to you in timelines and graphs, so you can check you're not peaking too early, or too much, or whether your plot-lines are "tangling".
This isn't the software giving its opinion, it just represents what information *you've* assigned to elements. Great for getting the pace evened out and spotting plot weaknesses.
As you can no doubt gather from my gushing, this is a powerful piece of software if you want it to be. Me, I tend not to use these more "power" features so often with short stories, but on bigger projects like novels, I may turn to them to get an idea of how it is flowing from a "mile-high-view".
Great for keeping tabs on characters, ideas, etc. with a full array of features such as character lists (allowing a lot of detail to be recorded, again, only if you need to), premise, backstory, and more. You can view your story in several ways: index cards (in various ways), Act>Chapter>Plot Point (Gestalt View), just plot points, or chapters, etc.
As I've said before, use it as much you need to.
Sometimes, I just map out some plot points, get inspired and get on writing the actual story. If I get stuck, I go back to Power Structure, retrospectively fill out what I've done so far, maybe put in some plot points for where I want to get to, and find it very useful for trying to get past the "sticking point" as I can get a better overview of the story and play around with it easily.
This is it's strength: it doesn't dictate how you should write. It lets you write your way, and lets you use it in the way that suits you. This means you can dip in and out of it without finding yourself forced into a corner by what it thinks is best. I use it in different ways for different projects. You can even change the terminology it uses, if it doesn't suit you.
It's worth demoing and sticking it out. I bought it back in 2002, and after getting over the wealth of information it can store (my nature meant I wanted to fill out every field which just meant I never wrote anything!), found it to be an excellent tool, and worth the price I paid - although it may be slightly over-priced depending on what you need (hence one star off on the price rating).
The strength it has over standard outline tools is that it is aimed at writers (without trying to tell them how to write) so you can just get on and use it, without having to adapt a generic tool to a specific task.
I also use CopyWrite when I'm writing the actual drafts, as this is great for keeping together all the documents in a managed environment with versioning. Power Structure walks all over it in terms of its ability to structure and manage the actual story, though.
(Also, Power Structure's developers have a Windows product called Power Writer that is planned for Mac release. This directly competes with CopyWrite, and as it integrates with PS, means you can access all the information regarding characters, plot points, act structure, etc, within it as well. Very exciting prospects!)
I then use Mellel to prepare the final draft with all the headers, page numbering, etc.
I just hope they update its look soon, as I feel people may not give it a second look, simply because it looks dated.
Maybe they'll overhaul it if they get round to a release of Power Writer as well.