Of course, this program is only free to an extent. If you only want to poll 30 or fewer people, the program is free, but if your audience is larger, there is a monthly subscription cost. For most class sizes, a free subscription would probably be all that was needed.
This seems like it would also be a great way to get feedback and opinions from people in class who may not be as outspoken as others. Or in classes that talk about current events, students may feel concerned about voicing unpopular opinions. For example, some students may feel more comfortable expressing their opinions about controversial topics (abortion, international politics) anonymously.
And classrooms are certainly not the only place where this tool could be used – I’m thinking of creating a poll on library usage!