Sure you can use your fingernails, but if you're looking for consistency and real crisp folds, you'll want to use the EASY FOLD. We use the EASY FOLD for every example book we create. On average you can fold about 10-15 books with one tool, after that the tool wears out a little and you're better off replacing it.
The EASY FOLD has been designed to our specific needs as a bookfolder. During the last few years we've developed and improved the way we use the EASY FOLD.