It depends on when, where and why I'm riding. On my winter commuting bike (Muirwoods 29er), I use 700x50. On my cross bike (Jake) I use 700x28 for commuting, 700x25 for centuries, 700x30 knobby for cyclocross races. I've got 700x25 on my fixed gear Cross Check right now, but I've got my eye on some wider tires for the winter. It's nice to have the option.