I recommend whiteboard apps for classroom use. Pre-recorded lessons can be especially great for ESOL students, as they can watch the lesson as many times as they need to without the teacher having to duplicate their instruction time.
-ShowMe integrates with social media
-has a rich library of free content
-controls are simple, making recording easy
-you have to have an account, or else you can't use the app
-video can't be stored to the camera roll, unless emailed to yourself
-no support for multiple slides
The bottom line:
ShowMe is a solid whiteboard app. If you like the controls, and you don't mind having to setup an account, go ahead and grap this whiteboard app for your iPad.