I'm the creator of Calendar.txt. I'm very happy to see this on Hacker News.
I've used Calendar.txt as my calendar for over a year (and previous versions before that). Having previously marketed both digital and paper calendars, and tested the digital variety, I felt they were excessively complicated.
For me, the benefits of calendar.txt have been reliability, simplicity and integration with established Linux tooling (grep, git, scripts). The downside has been lack of automatic synchronization, a tradeoff for simplicity and reliability.
If you want to ask about using Calendar.txt, I'll check here during breaks from my Django intensive course.