I have an A4 page a day hardback diary but I don't use it as a diary. Inside the front cover is year planner with a box for each day. This is used as an index, anything I'm supposed to remember/know about gets a day and that's the page I write my notes in.

Far more importantly, I get to stroll around the building with a big black book. It's handy for slapping down on someone's desk to announce your presence. If you're being hassled for something you've forgotten to do you leaf through, slam it shut and deny all knowledge.

I wouldn't be without it. :-)