Finally, something that I can answer.
I took a gap year after high school, which I spent in South Africa and Uganda.
I was incredibly burned out after high school and didn't know what I wanted to study. My family is from South Africa, so my folks were very supportive of me going and contributed financially toward it, which was an incredible blessing that I am very thankful for. I understand individuals paying for their own travels (and I did pay for some of mine), but my parents could afford it and I lived very inexpensively (280/month for all living expenses, staying in a commune type place).
The benefits: It was an incredible experience. I learned a ton about people and myself. Some of my roommates came from impoverished backgrounds, which was very eye opening. I worked in an international community while I was there, which not only taught me a lot, but is an excellent resume piece.
The negatives: I had some challenges being so young (I turned 18 a month before I left the states). Some of my greatest learning experiences were from some of the crap that I dealt with (shady ethical standards, misogynistic coworkers, etc). Since my family is from South Africa, there were some family friends that lived nearby so I was able to go to them for emotional support.
Would I recommend a gap year? A thousand times yes. It was a difficult year, but I came out better because of it. As soon as I got back to the states, I enrolled in community college and eventually graduated from a state school (even earned an outstanding undergraduate award). Within three months of finishing, I was gainfully employed in a field related to my degree.
My advice is to be careful with any travel to third-world nations. Do your homework with any volunteering groups and talk with people who have been there.
Also, gap years should be productive. Learning, gaining new experiences, growing as a person.