The back housing is all one piece now, which will make it harder to swap b/c you've got to remove all the little parts inside now.
Aluminum, as we all know, will dent. But it will not shatter. Which would you prefer? I actually prefer that it shatters, and that may put me in the minority.
The aluminum that apple uses in the iphone 5 is softer (and cheaper quality) than the "good stuff" according to some ridiculously knowledgeable people on this forum.
In the iphone 4, it was glass and could shatter easier, but not really dent.
But replacing the back panel was a 2 minute job and $7 shipped (in your color of choice) on ebay.
Personally I feel the iphone 4 is made better than the iphone 5, and if replacing the back panel costs me $10 or less each time it shatters, that's still way less than replacing the aluminum housing once (b/c of the likely labor costs it will involve--ordinary joes aren't going to be able to remove all the little pieces of an old housing and put it inside a new one), and it looks brand new each time.
What I'm saying is that people don't really care what a phone is made out of as much as they care how much it will cost to fix if it breaks/cracks/dents. In this regard the iphone 5 is a step backwards--it's going to be harder to repair and likely costlier b/c it's going to be harder for the average person to swap out the back.
Now if you care about replacing the screen, it may be easier to replace the iphone 5 screen since everything comes in as one piece and may be easier to access. Probably most people care about the screen replacement more than the back side replacement, but with the scratches and softness/bendgate of the unibody, there are going to be alot of unhappy campers when they think that they can replace that dinged unibody in a year with ease.
I'm fairly certain that the iphone 5 is going to be my last apple smartphone, but it does tick all of the boxes for me and the foreseeable future.