I have had my eye on the Shuttle boxes for a while, and someone showed up at our lan party with one, they are very nice and sleek and still offer a high degree of upgradeability - but will never be as small as a cube.

While I like the size of both the Shuttle and Cube, the biggest advantage to me was the noise. The Cubes only source of noise is the hard drive. (this is the same for the last gen CRT iMacs) The Shuttle on the other hand sounds noiser to me then my old tower if I run CPU intensive apps, since it sits on my desktop, and does not have the worlds best fans inside it.