I've had lots of pen-pals over the years, from when I was about 10 to into my 30s. I've found it very rewarding.
You learn about people in other cultures, and sometimes make great friends for life. Some of my best friends in life started out as pen-pals. You also train your writing skills.
I think it's a great thing for people who enjoy writing. On the other hand, if you're more of a "doer" in life and long writing bores you to no end, then it's probably not for you. If it feels like a chore, then it's not for you.
We're a bit past elementary school, but I helped set up a pen-pal exchange between two elementary school classes, and the kids had a good time writing a bit about themselves and even more fun reading what their pen pal said about himself or herself. I don't think any enduring friendships resulted, but even temporary pen-palsships can be rewarding.
Unless your pen pal turns out to be a stalker or somebody who wants to borrow money, there's probably not much of a downside.
I find these forums to be a type of group-pen-pal experience.
I had a pen pal from virginia for a few years during elementary school.
It was fun...and eventually we both got that crazy new fangled thing called 'email' and 'AIM' and started talking on there. I don't really know what happened but it sort of just stopped as we got older. I can't remember what his last name was though...Eric....something. I'd love to figure out his last name and facebook him.