First, quality. Once installed, a Brodit mount looks like a factory fitment, and is rock-solid. The look and feel is worlds apart from a cheap mount.
Second, flexibility. Brodit takes a two-part approach to mounting systems: the vehicle mount you attach to your car (which Brodit calls the ProClip), and the cradle which holds your device. By mixing-and-matching these, you can attach pretty much any device to any car, and put it right where you want it.
It also means that when you come to update your phone, you only have to replace the cradle that holds the device: the car mount remains in place. Conversely, if you change your car but keep your phone, you can simply replace the car mount.
Having used Brodit holders for various iPhones (and cars) over the years, I echo Ben’s choice of Brodit for being the choice for the most solid, factory fitted-looking iPhone holders on the market.