The amazing aspect about flying to Tainan is that the flight takes only 45 minutes. And, since domestic flights leave from Taipei's Sung Shan Domestic Airport, which is in the northern part of Taipei city proper, and at most a 15 minute drive away from the city, one can quite literally arrive 20 minutes before the flight takes off, walk up to the counter, purchase a ticket, and then board the plane! There are flights almost every hour, run by one of Taiwan's four domestic airlines (yes, there are four domestic airlines in a country about the size of West Virgina). So flying down to Tainan is as convenient as taking the train or bus from Boston to New York.
I was also quite relieved to find that the planes used on this route was a nice sized plane, with room for an actual first-class section, and not a rattling, smoke-spewing one where the passengers are seated according to weight.