I like the metal look of the newer model, as opposed to the plastic look of the previous one. The old models got smudgy fast, and the metallic finish looks less prone to smudging.
When it comes to the look of the interface, though, I actually like the 5g iPod better than the 5.5g or the 6g Classic. The old UI was relatively smooth and easy to work with for me, but the 5.5 saw a lot of glitches and short hang-ups that made me too often wonder if I had pressed a button or not.
I went down to the Apple Store today and checked out the new iPods, and the new interface just doesn't cut it for me. It's less responsive, and the glitches and hold-ups are still there. All I want is to do is listen to music, and I just don't need Coverflow or half-screen album art or even a search function.
It seems the iPod interface has become bogged down with extras I don't need, at the expense of smoothess and speed with I do need. The new iPods will score high with most users, but not with me. I want the basics and not a lot of overhead to kludge up the system.
I played with the iPhone when I was down there today, and I have to agree with the marketing hype that the iPhone is the best iPod. The iPhone music interface is smoother and easier to navigate than the newer iPods, so I'm leaning strongly to the iPhone, or maybe the iPod touch, as my new music player.