I'm evaluating it right now and hope to post a review on my Tablet PC blog soon. It's actually a very clever application. I don't mean this to be thorough so take it for what it is - a quick opinion based on a few minutes of playing around with it. What it does is provide a capture space where you can jot down "stuff" as it comes up, and then, when you're ready to process, it "files it" appropriately as a Next Action, Someday/Maybe item, Project, Reference, etc.
It retains the "inkiness" of your pen input but pre-recognizes it so it'ssearchable.It can import your day's task and calendar list from Outlook as well as anything in your primary Inbox too.
For a first cut at a Tablet-focused GTD implementation, it looks very promising.