I would say every house is definitely different on the way the heating and cooling ductwork is set up. In my house one room is just above the furnace so it is the first feed off the main Plenum. That room definitely benefits from a smart vent. When that room is closed I do notice more airflow coming out of the vents at the end of the line. As for energy-saving I don’t think it does much at all, but it definitely evens the temperature between the rooms. The best part of smart vents is they can work with other devices in your home, so if a guest room is not being used the vents will remain closed. I suppose that could save some money but it would be pretty hard to calculate and see if it is saving you money.
And one thing people worry about is pressure build up in the system. Many people worry that if too many vents are closed it could damage the furnace motor because of pressure buildup. When I had my furnace replaced I brought my concerns to the installer. He said he could build a chamber under the furnace to help relieve static pressure buildup. Now the motor can flow freely if too many vents are closed.
In the end all my rooms maintain a set temperature except for the basement, cold air will always drop. So in the summer the basement is always much cooler.
Hope this helps