Here are my impressions of the iPen.
Stuff I don't like:
First of all, the 'palm rejection' doesn't work at all. Resting your hand on the screen results in random lines and/or zoom commands and whatnot. Ignore that suppposed feature, it's just hype.
The calibration issue is real. I don't really mind having to calibrate when starting up an app, only takes a sec, but the calibration is iffy. Sometimes you get decent accuracy, sometimes you don't. Sketch Club is pretty good at it. Goodnotes is horrible, it never calibrates close enough to use.
Even after calibration, the distance between where you draw and where it appears is variable. It can also drift during use. I didn't find that Iggy's suggestion of drawing with the dongle on top helped. In my case it made it worse. I don't know why
Stuff I like:
You can draw a very nice smooth line, with less shake than you normally get with a finger or stylus.
It doesn't start drawing until you intend, because you have to press down. This is much more like a normal pen, which doesn't draw unless it's touching the paper (unlike a stylus or finger which can draw while hovering).
It's a niche product. I will probably use it for some line work as needed. It is not good as a general purpose drawing/painting tool.