I always argue we should not give to the participants the handout before the class, because they will focus on the handout instead of listening if they have the handout. However, the downside is that:
Their attention is drawn to another task - taking note;
There is so much they can write down in a class and may thus miss to capture some learning
Since they take note on paper wherever convenient to them, some could easily lose their note and thus learning
A solution which I learnt in the conference from a number of speakers is what I call the interactive handout. Basically, the handout contains some but not all the class content. More importantly, participants have to do something in order to record the key contents. It could contain fill-in-the-blank or space for drawing.
Of course, you need to spend extra time to prepare this (instead of just printing the power-point copy). Well, what good thing does not require preparation