Folder, with plastic sleeves for hand-outs and some loose-leaf for note taking (I file it after in subject groups). For Maths however, I use a book like everyone else.
I've always been an advocate for books but I tend to spend too much time trying to make my book look neat and organised, which became a problem in senior years where you just don't have time to make everything look perfect. With loose-leaf, you don't have to be as neat because you know you can always take it out and rewrite it on a seperate sheet if it is absolutely necessary to do so. I reckon this is probably the biggest advantage of having loose-leaf, you can easily take one section out and work on it (add notes and so forth), as opposed to books, which is fairly fixed and static - especially in regards to rearranging your notes into chronological order (serveral my teacher jumps from places to places).
Also, you don't have to carry around as much. The fact that I usually have 5 or more subjects, out of a total of 6, means that the weight of carrying books becomes significant. Although I use a folder and everything, it's still very heavy considering the number of textbooks I have to carry around. By using loose-leaf, I can take just enough for the day, without having any excess.
I'll admit the disadvantage is that they can become lost (though this has never happened to me) and that you don't put as much effort into it (since it's not something like a book, which you try to keep as organised as possible). But yeah, I reckon the advantages outweight the disadvantages.
