It’s not glamorous. The couchette compartment sleeps six, the mattress is thin, and the border crossing wakes everyone around 2am for a passport check. But it drops you in Zagreb by breakfast for less than a third of the cost of a short flight, and you skip an airport entirely.
Book the compartment, not just a seat, it’s the difference between actually sleeping and sitting upright for seven hours. Bring your own water and something to eat; the dining car closes earlier than the schedule suggests.
I’d take it again over a budget airline any day, mostly for how normal it makes crossing a border feel.