Pen/Pencil and Paper for serious equations will beat any I/O interface anyday. Even tablet recognition pales in comparison, though direct tablet drawing might come a close second. I do this for all the notes I need to take, and as an added benefit, I'm forced to review my notes if I want to enter them onto my computer when I have more time. Plus, you can absorb the lessons better if you're not focused on the details of entering long equations into your computer.

Was it carried by those terrorists we let in to fly planes into those buildings? ;)