Quick and Easy, simple formative assessment tool.
The immediacy of the tool, and low tech threshold are both a huge plus. It is great for a couple of questions, but really no more than that as you aren't able to ask for more than true/false or multiple choice.
How I Use It
I have used plickers with my math students to do a quick snapshot at the outset of a class to determine if my students have the requisite prior knowledge for a lesson. It also works well as an exit ticket. It is limited, as you must have either multiple choice or true/false questions.