As a former Amtrak employee, all I can say is the sleeper rooms are very crowded, food is getting worse (check to see if the train you are going to be on has only microwave food or an actual diner with cooks), and trains are often late, at least on the west coast. Don't do it if you are in a hurry!
Some people really enjoy it; others don't. It is easier for children though as they can easily walk between cars and aren't as cramped as on a plane.