I used in back in high school, and have just started using one again a few months ago. I just write down what I did every workout, and keep a food journal.
I like it, cause you can always go back and check what weight and reps you need to hit a PR, and for the first time ever, due to my food journal I'm able to tell what works best for my body nutrition-wise.
I don't carry it around at the gym. I think that's stupid. It's hard to remember what you did an hour earlier, LOL. I keep it at home.