I have one. I write in it like I'm talking to a friend, then cringe when I go back a few months later and look at some of the things I wrote/thought. Keeping a diary/journal is a really great way to vent without actually having to speak.
I used to keep a diary all the time when I was little but I stopped because my sister would always find them and make fun of me. I had one a few years ago but it got me in trouble because the person who found it didn't like what I had to say about them (I don't know what they expected after treating me like shit) I started another diary last year but I soon got rid of it because the things I wrote made me cringe and I didn't want anyone finding it.
I have a ton of diaries - but I don't write in them as much as I used to. I used to write all the time when I was younger but then I started writing stories instead... I would like to start writing in one again though. I love going back through old diaries and laughing at myself haha
I used to have a diary when I was younger, but I think I got rid of it after middle school. I remember looking back at it and just laughing because they entries were so silly!
I have a journal type book thing. Not really a diary. More closer to what Nikki Sixx described his diary as: something that he could look back on and remember what he did the day before, or if he died, a nice little suicide note of his life. That's how I feel about mine. It helps me keep it together. Helps me remember what I did the day before. If I die, it tells a nice little story of what I've been doing.