I'd just like to point out that in HP's second year, when he's dueling his arch rival, Draco, he uses the spell to try and tickle his opponent in front of the whole school... Sounds smart! Not like Draco is one to try pulling a snake on you or anything!
Harry Potter is one of my favorite movies. I think it's so popular because it's beyond of people's regular imagination. The concept of the movie is new and fresh to people so many get curious about it and want to watch it more.