I tried out that Paper app. The interface is very nice and there is a lot to recommend. When I saw that they were charging £1.50 just to add a single pen to the app I just got annoyed and deleted the app. With so many excellent apps out there for 69p I can't see any reason to pay about £7 just to get a single app fully functional.
Maybe if my art stretched to anything other than one headshot of Superman I'd have felt differently.
Yeah, the In App Purchase model is a bit controversial. It all depends on how you look at it, though. I only paid for the pens I'd use and the overall cost seemed worth it. I might go back someday and buy another tool, but at that point I think the overall cost of ownership will have proven itself out.