I'm an (almost) 100% paperback person here. Partly the cost, which is pretty high, especially if you start collecting some of these series' that never seem to end, but also manageability. Take a Peter F Hamilton as an example - each book well over 1000 pages, doorstops in paperback, totally uncarry-able in hardback.
I tend to have an ongoing wishlist of older books that I will keep an eye out for in charity shops, but for many things it's often just as cheap to get brand new copies online. Also, being a book lover I do like my copies to be in reasonable condition and torn covers and scribbles make me unhappy so I probably wouldn't get them second hand online as I can't see the state of them before I buy them. The only exception I make to this rule is for reference books as they are so ridiculously expensive and often older versions are better than the revised ones.
If I'm desperate to read something as soon as it comes out (rare as there are so many other things I still haven't read) then I make use of the local library to request/reserve a copy as soon as is humanly possible. The library is definitely my friend :-)