With the risk of sounding like a sour guy, I'm just going to say it: why do people pay money for apps that are essentially web browsers in a shiny package?
I am not against people making money by providing better access to content that is available for free (who hasn't paid for the odd Twitter app), but just take a look at the list of features on the app's promotion page:
* View top submitted stories, optimized for iPhone
* Explore comments and threads
* Browse web links without quitting using the in-app browser
All of these things are true for browsing Hacker News in mobile Safari as well. That's hardly enough to earn the predicate "insanely great" or the $3 price tag in my world.
Admittedly, using Hacker News on an iPhone is not the best mobile browser experience (it could certainly use some css @media queries and perhaps a meta viewport tag, essentially few minutes of work would do the trick), but that hardly warrants a native app, especially if it's not free.