Educreations app as my primary screencasting app for my students to create screencasts. Educreations is a FREE app and has, by far, the best writing style of any of the whiteboard apps that I've seen in terms of writing accuracy and colors. It is also very easy to upload images (such as graph paper for math) to notate on from DropBox or the photo library.
I also like how easy it is, as a teacher, to watch my students' screencasts. With a little help from other teachers over Twitter (thanks @gregkulowiec and @MsMagiera) I determined the easiest way to set up Educreations on my student ipads was to have my teacher account logged in on all of the iPads. Then, I taught my students a uniform naming convention for their videos, which are all uploaded to my teacher account so that I knew who submitted each video. I then can watch the videos from my computer and/or project or post their videos for other students to see! So far, it has been very slick!
I think a fair barometer of technology in the classroom is to measure whether it allows students to demonstrate understanding, create meaning, extend learning, and/or to create something that can be used in the future. Using screencasting seems like an outstanding way to reach these goals! I also love the metacognition that occurs for students as they work through the problems (I ask them to narrate as though they were teaching someone who missed class the day we covered the problem they are working).
Below are 2 examples of screencasts my algebra students created. In class today, my students were to select 2 problems they missed on a recent test and correct their mistake(s) using the Educreations app to demonstrate their understanding and correct their mistake: