This is a sore spot. First off 53 seems to have been holding off support for other devices while it was working on releasing it's own pressure based stylus. Needless to say many users are displeased that what should be generic hardware ends up tied to working on some apps, but not others, because Apple has not standardized on this yet.
I like the idea, and it's a slick build, but the idea of single app hardware ( or vastly limited ) is a real turn off. Buy it if you need it, but I expect that this will be a very limited device in terms of longevity.
No denying all their products are slick! Overpriced, but slick, like the $40 moleskine book they sell. The bigger issue with pencil is it looks like the tip is no better than something you get on a $10 stylus, and has no pressure sensitivity options.