Syllable really looks promising, but the last time I used it (a few months ago) it was really unstable. It is lightning fast though (boots in 6 seconds on my 1,66GHz box), and has some nice features.
I don't think it's ready for the mainstream just yet. There is no 3D (there wasn't the last time I checked), and it has only few applications.
I've heard that it was fairly stable, but not very complete... Still USB support has been more or less finished (low-level support of the sort needed to allow programs to talk to Scanners, Non-Mass Storage-Enabled Cameras and other stuff is now in place, while support for standard devices like mass storage, keyboards and mice has been there since 2003) 3D support is (sadly) lacking, but it's not a big priority... Frankly, I'd say Syllable is now the point where writing programs for it might actually be useful. As a photographer, I'd personally love to see a nice photo manager with good basic editing functions (aka an iPhoto or Picasa type program) Anyone want to code it for me?
Why not try to port F-Spot and GIMP over? You would have some photo thingies...
Looks quite nice. Am downloading it, I will try it out on VirtualBox. I won't consider it to be too useful or well supported though.
It looks like a nice piece of software. It could get popular if it has a great community, is open-source, free, and has lots of features.