From what I've used of it, waifu2x works best on things with really solid linework and colors. So vector graphics, your typical anime screenshots, most fanart, it will all work fine. If you have art like >>2078561
though, waifu2x will wreck it. Even if you turn off noise reduction, it will still try to smooth out all of the pencil shading and you end up losing part of what makes it nice.