I have several of the pay-for-use productivity apps mentioned, and my experience was like the parent comment said. Initially got the app to fix a problem (content creation/editing is difficult on an all-touch device). But just fooled around a little and never really got engaged. I still do that stuff on my laptop.
My take-away was that I wanted precise pointing and a keyboard and a big screen for those tasks, because I'm fussy about the results.
But like a lot of the HN audience, I'm not very typical, so my experience may mean little ;-).
Regarding payment model, payment wasn't the issue. It was functionality.