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.