I had to take a CPR refresher every year and was an instructor at one point. I've had to use it quite a bit over the years, and it's not fun. CPR on a dummy is quite different than on a real person.
Gato's right, it's not as easy as it looks. You start off well but after time, you get tired and it's not as effective. That's why one of the first things you ask is if anyone else knows CPR so they can switch with you.
My parents watched their best friend die in front of them because out of a full tavern, no one knew CPR. It takes a few hours to take a class and you can most likely find one for free.
One of the 10 year old girls that I taught in a class saved her mother's life less than 2 months after taking the course by giving her the Heimlick Maneuver. There's no question that it's worth it to learn, but so many people never do.