Handling and brakes come first on my list.
Then performance (no good having loads of power if it won't corner or stop?)
Looks come last (e.g. styling, whats the point in having a car that looks like it's doing the ton standing still in the car park but has a lump under the bonnet that would'nt pull the skin off a rice pudding?). As has been pointed out the IS is a good looking car anyway, that's what started me digging back when I bought it.
Personal preferance is to have the car looking as standard as possible, keeps chav's and the law off your back, but goes like **** off a chrome shovel :D