It's a great way to get into comics and also a great way to encourage younger kids to start to enjoy reading in general. Comics are much more mainstream now since films like Watchmen and Iron Man have come out but a lot of people still wouldn't dream of setting foot inside their local store. More fool them.
I got into comics when I was about 13 or 14 after seeing The Crow. It was an adaptation of the James O'Barr graphic novel of the same name and in my opinion is still one of the best comics out there. When Gerard Way unveiled The Umbrella Academy to the world it was through Free Comic Day and I thought it was such a cool concept and nice way to give something back to the comic community.
Kevin Smith (director and comic geek extraordinaire) and the gorgeous Milo Ventimiglia have made these little videos for your viewing pleasure about the day.
So go get involved, pick up one of everything and take a chance. You never know, you might like it and everyone loves a freebie.