I think using the IPad for Life Drawing works well. I was able to get the figure done in the 1.5 hour session, and I did the chair and background at home.
I used the Art Set Pro app on the ipad, because it behaves very close to natural media. I like the color pencil for drawing, and for coloring and shading I like the oil pastel, and paint. I manipulated it with the eraser, rag and water tools. I used a Wacom Bamboo Stylus.