Back in 2004, I signed up for the Google Code Jam for the first time. Unfortunately I didn't make it past the qualifying round.
I was a bit luckier in 2008 and 2010, making it to round 2 both times. In fact in 2008 as I recall I was one of only two participants from the Philippines who made it to round 2, which allowed me to jokingly brag about being the #2 programmer in the country.
I registered again a few times after that, and tried again last year (2016), where I only made it to round 1.
I'm no stranger to programming competitions, although the GCJ was the only one I've ever really participated in. Back in college, one of my friends usually joined such competitions, and I had a good time going over some of the problems with him.
Programming competitions don't usually reflect the kind of programming done in most real-world jobs (although it might be interesting to use competitive programming problems in whiteboard interviews and see what happens). They focus on pushing the boundaries of the programmers talents in problem solving. You also need to be fairly familiar with algorithmic approaches to searching, sorting, and so on. Having a decent background in math also helps a lot.
The Google Code Jam in particular focuses on scaling up your code to handle very large amounts of data -- well past the integer limit sometimes. Each question usually has a small and a large input. For the large inputs, you only have a short amount of time (I think it was 5 minutes?) to submit the output. Presumably you wrote and tested your code before starting the timer, so the short time is really only for your code's execution. You can afford to be inefficient or maybe even brute force your way through the small input, but if you want to solve the large inputs on time you have to be able to optimize your approach.
When I first signed up, I dreamed of winning the entire thing (and of course being able to brag about being the best programmer in the world). But realistically, a lot of the people joining these competitions are "pros" more or less, so the chance of a "casual" like me winning are not that high. They are "pros" not in the sense that they do it for a living, but more of they do a lot of these competitions and regularly practice for them. Websites like TopCoder provide such competitions regularly for those who are interested. Also, I feel like I am very old and rusty, having worked too long in web development stuff where my focus is usually on a different kind of problem.
Still, I think it's a good idea for programmers of all skill levels to give it a try, just to see where you are at. Who knows, you might even make it all the way! The next Google Code Jam starts soon -- the qualification round is on Apr 7. I signed up, although I'm unsure if I will have the time this year even if I advance, due to scheduling problems. But a man can still dream, right? Who knows, maybe I'll see some of you there too in the final round!