Thanks for the replies. I still don't know what to do.

I have a normal notebook and a desktop computer. A small notebook could be quite useful, but (for some reason) I would like to write my short notes with pencil not with the keyboard. When I read paper articles I usually write short notes on them or have a paper notebook where I write short (sometimes incomprehensible) comments. After finishing the article there are usually pieces of paper everywhere with weird references to different papers! So the annotation function of prs-600 could perhaps be what I am actually looking for.
Has any of you tried the IRIS pen? If it works, it could be very practical.