My students were very excited and engaged when I first introduced Voki's to them.
I differentiated the instruction for my math classes by having them pair up to create their own Voki's. I wanted the students to summarize the main concepts in the math unit we have been studying to allow them to revisit some materials and also provide a bit of review for them. This was the part of the lesson that was differentiated, I assigned different concepts to different students. I assigned the math concept that I knew each grouping would be successful at summarizing. These Voki's were then presented to the class and posted to the class website. Students at home could then click on them as many times as they wanted and they would hear the specific math concepts explained over and over again.
You can make a Voki here.