I'm currently at NDC London and at the end of each session they use a traffic light system to provide feedback. If you liked a session you put a green card in. If you didn't like it or thought it was bad you put in a red card and if you are somewhere in the middle you put in a yellow card.
Here's the thing though. With multiple sessions on at the same time, what are you saying by staying to the end and then putting in a red card?
Yes, it's good to provide feedback to the speaker and organiser if you didn't think the session was very good.
But why would you stay to the end of a session you are not enjoying or don't think is very good? Vote with your feet. If you're not enjoying something or benefitting from listening then go to another session.
If you stay to the end and leave a bad review aren't you saying something about yourself in addition to what you're saying about the session? What does it mean to say "I didn't find this useful or enjoyable, but chose to stay and listen to it all"?
Additionally, providing feedback via a voting system is good. But providing more detail is better! Whether your feedback is positive or negative please share it with the speaker and organiser. Events are organised and speakers speak for the benefit of the people who attend and listen not for the ego of the speaker. By providing constructive feedback you can help the speakers do a better job in future and allow organizers to ensure that session abstracts match what is wanted and delivered and that it benefits the audience and matches their expectations.