I did one! I started it the day I did my pregnancy test. I love looking back on it now that my boys are 4, especially from the day we discovered we were expecting twins! Sometimes I'd write pages and pages, sometimes just a few lines.
I still write in it now, I used to write every month to tell them all about their development and once they were born all their progress. I write in it when 'special things' happen - like Obama being elected etc. It's great for recording all their funny little personality quirks etc - I've forgotten so much already so I can't imagine how little I'd remember when they're adults!
TBH I had thought I was going to have a girl so I would be able to give her the books when she's older....I got 2 boys who probably won't care when they're older - hopefully their future wives will like them instead!!! Lol