I've tried both, for the Sweden<->Germany route. I've usually been disappointed that the train was more expensive than a flight. The train has much fewer luggage restrictions, but lugging lots of luggage around when changing trains is also a hassle, while with flights they do that for you. With 4 hours of travel, you don't need to worry about food. With 15, you do. Though you can bring a full meal, including drinks and a knife, on the train. Plus, there's AC power for the laptop.

My break-even point is at about 6-8 hours by train, vs. a plane. That gets me to Hamburg.