dc1,
All that stuff printed on the knee board that the brochures tell you about is rubbish. You'll never use it. If you need to use it, then you probably haven't studied properly.
Having said that, I'm surprised to see so many people here saying that you should buy
A) an ordinary
b) A4 clip board.
I would recommond an A5 pilots knee board.
Firstly because I find my A5 actually takes up too much space as it is! Perhaps that's because I'm not very tall, and with the seat as far forward as it goes, I don't have a lot of space. However I think that an A4 is probably too big for most cockpits that you are likely to be flying unless you progress onto much bigger machines.
Second reason is exactly as Mike Cross pointed out. Writing on a standard clip board is a two handed job. One to hold the board, and one to write. Fine for taking down taxi instructions etc. when you are on the ground, but not great for takeing en-route times and clearances when you are in the air. Neither an instructor nor an examiner will be impressed with you letting go of the controls to start to write! A pilots knee board generally has some way of attaching to your leg. Mine attaches with velcro straps. Therefore I only need one had to write with, while the other remains on the controls.
I could see that you could use an ordinary clip board if you didn't have anyone in the other seat...you could leave the board there, and lean on the seat, but most of your training will be done with both seats filled.
Go for the proper A5 pilots knee board, that attaches to your knee.
dp