I just had a look at the Circus Ponies Notebook, and you guys are right: it does look like a great app. At that price, though, they should provide a strapped down lite version so that one can give it a try without laying out such a big sum just "on spec"!
Not disagreeing, just noting a problem developers have with these kinds of apps. From what I can tell, the Circus Ponies Notebook is a full scale analogue of Microsoft's OneNote (the application cited by the OP and a VERY popular application in the Windows world.) Further, as guthrien notes, the Circus Ponies Notebook is a popular Mac Application.
So how should it be priced? According to the median price for tablet apps (about $5) or according to the functionality it includes? That becomes an especially difficult problem when the app more or less provides the same functionality on the iPad as on a full scale computer. And if a developer decides to provide a "lite" version, they face the question of just what features they include (and block) in a highly integrated application.
The basic problem, it seems to me, is the entire business model Apple insists upon for apps in the App Store. Apple takes 30% of the purchase price and doesn't provide an obvious, easy way to return an app simply because one decides that while an app performs as expected, its value for an individual user simply doesn't measure up to the price paid. That's a relatively minor problem when an app costs less than $5; it becomes a more important issue when the price is five or six times that.
I've read anecdotal accounts from developers who claim Apple encourages them to increase the price of their apps. Obviously, that's in Apple's interest (Remember the 30% cut.) But unless Apple is prepared to support a simpler, more obvious, and more liberal return policy, consumers face exactly the dilemma Heaviside (and I) faces.
Personally, I'm happy to pay for functionality and well-designed apps. And if Apple provided a "no questions asked" return process that I could use easily (perhaps only for apps that are priced at more than $10), I'd be more prepared to invest $30 (or more) in a sight unseen app.