Aluminum does provide a premium feel no doubt but that's where it ends.
It's conductive so its very important the electronics within never come in contact with it. It's also important you don't touch it to a ground while holding it due to electrostatic, however I haven't heard that being an issue.
It blocks a lot of RF so when the day comes that the iPhone adopts NFC, wireless charging or any low power source transmission they will do away with it or design something of the exterior of it that's plastic.
Aluminum in electronic devices is also structurally low grade. It's nothing like cast parts you'll find on an automobile. Or an aluminum alloy you'll find in things designed for abuse, like a gun frame which I might add is still prone to cracking. That kind of stuff is transitioning to plastics like the handgun in my avatar. So just because its "aluminum" doesn't mean its an expensive high quality material. I'd say its closer to food grade like you would find in a soda can.
Also the coating on the iPhone is very very poor. There are aluminum coatings that are designed for the most vigorous torture test, used on weapons the US military carries. Other manufactures use a much higher quality coating like HTC and Motorola on aluminum. I have a can of Aluma-Hyde that cost me 8 bucks and you could probably cost 1000 iPhones with a single can and that stuff it TOUGH.
Never mind a baked on Duracoat (brand) which would be virtually impossible to scratch off with anything short of a blade. So all this aluminum scratches is total BS, it DOES with a crappy coating or on an unprepped surface. These coatings are CHEAP, I do them at my house!
That said, its a great material (aluminum) for the iPhone. It doesn't have NFC or anything like that. And it feels very high quality. I prefer my iPhone 4S material but if I lost my phone I'd have no gripe replacing it with an iPhone 5.
Btw that's not a rant, just an observation from someone that deals with abusing different materials and coating them.
Edit: here's an example of a good coating versus poor.
The safety (worn part center picture) was poorly prepped or had a low grade coating. It's worn out from wear from my thumb. Notice how everything else its in relatively good condition?
Matter of fact here are the slide rails. This is where there is metal on metal contact in a very violent matter at that.
Very very light wear.