I have been playing around with it for the last half hour or so and found it easy to use and potentially very useful.
What I don't know is how well it works as a collaborative tool (which is where my real interest lies -- there is plenty of good stand-alone mindmapping software), so I thought I would throw it out there for anyone to examine (Popplet makes this simple with an embed code. Yeah!).
If you are interested in trying it out, however, you will need to drop me an email (kwheaton at Mercyhurst dot edu) and I will send you an invite.
These kinds of open, collaborative tools that are easy to set up and quick to learn are great for classroom exercises; they are interactive and engaging. In my experience, students love them (If you are looking for another example, try Willyou.typewith.me)
It is also a great way to build a mental model of an intelligence problem. This app is in beta though and has no way (that I could find) to safely share or export the data. There is an offline reader application called Popplet Presenter which would allow a single individual to show his/her work securely (-ish) to others, I suppose.
I suspect these features are coming (offering these features for a modest price is the way most of these kinds of apps, like Mindmeister or Webspiration, make their money) but until then, this is probably best confined to the classroom.