I have bought and tested a couple dozen pens for capacitive devices over the past few years. The Dagi was the early winner (a Taiwanese company) and then came the Jot by Adonit (another Kickstarter project). I love my Jot and would like to request that BP integrate the SDK just has he has done with the iPen (which i have on order now to test).
FYI, i've read the thread on the iPen and I do have concerns. First is that it requires batteries (that last about 6 hours according to their site). Second, it requires an adapter. To me this is more stuff to break or lose. Is the additional accuracy worth it? I guess I'll find out. In the mean time, I suggest to all those out there not looking to drop $90 on a pen to try the much more affordable and very well crafted Adonit Jot Pro for $30.