Maglev or other high speed rail (by highspeed I mean 300km-400km/h, not this 1960s stuff California is talking about) is fine for dense cities between 200 and 1000km apart where there's enough traffic to justify the line (i.e. 200,000 passengers a day in each direction over the bulk of the line -- 1000 seats at 20 trains per hour in each direction)

For much under 200km there's starts to be little benefit to high speed (10 minutes - station design and frequency far more important)

For more than 1000km then you'll struggle to compete with planes -- although a 600kph maglev probably works at most scales -- certainly upto 3000km.