I got my blue unit today and the thing is huge. The blue looks much nicer in person, it's a mettalic-ish finish, a bit like the coloured iPod nanos. While the glossy finish of the original 3DS might look nicer, the new matte material feels much better, it gives a firmer grip and doesn't collect finger prints.
Build quality is a lot better and it feels much nicer in the hands than the original 3DS. The buttons are softer and the rounded edges no longer hurt after holding the thing for a while. The circle pad also feels much nicer and softer and is not as wobbly as the original one.
The screens are obviously huge in comparison to the original unit but the low res is noticeable at first, they simply don't look as sharp. That also depends on the game though, low res textures in OOT are more obvious while RE: Revelations looks every bit as impressive on the big screen, for example. When playing the difference is huge though, sometimes bigger is indeed better. ;-) 3D looks a bit better and more immersive to me, viewing angles are also improved, not by a great deal though.
All in all, it's a great upgrade and I would definitely get this one over the original unit if you're on the fence. It is huge though, keep that in mind if you really want a pocketable system.
Some pics:
