I bought the Cregle iPen before I decided on getting the iPad or the New iPad, but ultimately went with the iPad 2. I saw the videos on kickstarter and on youtube and it looked pretty impressive.
This past week I finally got my iPad in my physical hands and right away, decided to test out the iPen. I stayed away from Ghostwriter as everyone seems to detest it and I didn't want to pay for it

Plus from what I've read, it only has one calibration point so no thanks.Thankfully there was the Ibis paint app that everyone was talking about and I decided to have a go at it. Granted, I'm not the most skilled artist but it seemed to work decently, more better suited for writing than drawing but of course that app was designed for drawing and not writing

It just seemed iffy.
THEN, came along Goodnotes and the beta update for iPen. I must say the 4 main calibration points and the extra points *for your own comfort* made the iPen dramatically better! I've been using it awhile now and it works pretty damn good once the calibration is set. It works very fluidly; much more so than the Ibis and I would assume, Ghostwriter. At first though, I felt as though my handwriting was shaky, but I really believe its the preset tips the app uses. Overall, the iPen works very well with Goodnotes.
Here are some of my notes.